diff --git a/include/CLI/App.hpp b/include/CLI/App.hpp
index a287536e..085209d4 100644
--- a/include/CLI/App.hpp
+++ b/include/CLI/App.hpp
@@ -274,8 +274,6 @@ class App {
 
         std::string simple_name = CLI::detail::split(name, ',').at(0);
         CLI::callback_t fun = [&variable, simple_name](CLI::results_t res) {
-            if(res.size() != 1)
-                throw ConversionError("Only one " + simple_name + " allowed");
             return detail::lexical_cast(res[0], variable);
         };
 
@@ -293,8 +291,6 @@ class App {
 
         std::string simple_name = CLI::detail::split(name, ',').at(0);
         CLI::callback_t fun = [&variable, simple_name](CLI::results_t res) {
-            if(res.size() != 1)
-                throw ConversionError("Only one " + simple_name + " allowed");
             return detail::lexical_cast(res[0], variable);
         };
 
@@ -325,7 +321,7 @@ class App {
         };
 
         Option *opt = add_option(name, fun, description, false);
-        opt->set_custom_option(detail::type_name<T>(), -1, true);
+        opt->set_custom_option(detail::type_name<T>(), -1);
         return opt;
     }
 
@@ -347,7 +343,7 @@ class App {
         };
 
         Option *opt = add_option(name, fun, description, defaulted);
-        opt->set_custom_option(detail::type_name<T>(), -1, true);
+        opt->set_custom_option(detail::type_name<T>(), -1);
         if(defaulted)
             opt->set_default_str("[" + detail::join(variable) + "]");
         return opt;
@@ -454,9 +450,6 @@ class App {
 
         std::string simple_name = CLI::detail::split(name, ',').at(0);
         CLI::callback_t fun = [&member, options, simple_name](CLI::results_t res) {
-            if(res.size() != 1) {
-                throw ConversionError("Only one " + simple_name + " allowed");
-            }
             bool retval = detail::lexical_cast(res[0], member);
             if(!retval)
                 throw ConversionError("The value " + res[0] + "is not an allowed value for " + simple_name);
@@ -480,9 +473,6 @@ class App {
 
         std::string simple_name = CLI::detail::split(name, ',').at(0);
         CLI::callback_t fun = [&member, options, simple_name](CLI::results_t res) {
-            if(res.size() != 1) {
-                throw ConversionError("Only one " + simple_name + " allowed");
-            }
             bool retval = detail::lexical_cast(res[0], member);
             if(!retval)
                 throw ConversionError("The value " + res[0] + "is not an allowed value for " + simple_name);
@@ -509,9 +499,6 @@ class App {
 
         std::string simple_name = CLI::detail::split(name, ',').at(0);
         CLI::callback_t fun = [&member, options, simple_name](CLI::results_t res) {
-            if(res.size() != 1) {
-                throw ConversionError("Only one " + simple_name + " allowed");
-            }
             member = detail::to_lower(res[0]);
             auto iter = std::find_if(std::begin(options), std::end(options), [&member](std::string val) {
                 return detail::to_lower(val) == member;
@@ -541,9 +528,6 @@ class App {
 
         std::string simple_name = CLI::detail::split(name, ',').at(0);
         CLI::callback_t fun = [&member, options, simple_name](CLI::results_t res) {
-            if(res.size() != 1) {
-                throw ConversionError("Only one " + simple_name + " allowed");
-            }
             member = detail::to_lower(res[0]);
             auto iter = std::find_if(std::begin(options), std::end(options), [&member](std::string val) {
                 return detail::to_lower(val) == member;
diff --git a/include/CLI/Error.hpp b/include/CLI/Error.hpp
index 3e9f3b57..0758bb76 100644
--- a/include/CLI/Error.hpp
+++ b/include/CLI/Error.hpp
@@ -43,6 +43,7 @@ enum class ExitCodes {
     InvalidError,
     HorribleError,
     OptionNotFound,
+    ArgumentMismatch,
     BaseClass = 127
 };
 
@@ -146,6 +147,17 @@ class RequiredError : public ParseError {
     CLI11_ERROR_SIMPLE(RequiredError)
 };
 
+/// Thrown when the wrong number of arguments has been recieved
+class ArgumentMismatch : ParseError {
+    CLI11_ERROR_DEF(ParseError, ArgumentMismatch)
+    ArgumentMismatch(std::string name, int expected, size_t recieved)
+        : ArgumentMismatch(expected > 0 ? ("Expected exactly " + std::to_string(expected) + " arguments to " + name +
+                                           ", got " + std::to_string(recieved))
+                                        : ("Expected at least " + std::to_string(-expected) + " arguments to " + name +
+                                           ", got " + std::to_string(recieved)),
+                           ExitCodes::ArgumentMismatch) {}
+};
+
 /// Thrown when a requires option is missing
 class RequiresError : public ParseError {
     CLI11_ERROR_DEF(ParseError, RequiresError)
diff --git a/include/CLI/Option.hpp b/include/CLI/Option.hpp
index 477a9512..b3418d18 100644
--- a/include/CLI/Option.hpp
+++ b/include/CLI/Option.hpp
@@ -207,14 +207,15 @@ class Option : public OptionBase<Option> {
 
     /// Set the number of expected arguments (Flags bypass this)
     Option *expected(int value) {
-        if(value == 0)
+        if(expected_ == value)
+            return this;
+        else if(value == 0)
             throw IncorrectConstruction("Cannot set 0 expected, use a flag instead");
-        else if(expected_ == 0)
-            throw IncorrectConstruction("Cannot make a flag take arguments!");
         else if(!changeable_)
             throw IncorrectConstruction("You can only change the expected arguments for vectors");
         else if(last_)
             throw IncorrectConstruction("You can't change expected arguments after you've set take_last!");
+
         expected_ = value;
         return this;
     }
@@ -447,7 +448,11 @@ class Option : public OptionBase<Option> {
             results_t partial_result = {results_.back()};
             local_result = !callback_(partial_result);
         } else {
-            local_result = !callback_(results_);
+            if((expected_ > 0 && results_.size() != static_cast<size_t>(expected_)) ||
+               (expected_ < 0 && results_.size() < static_cast<size_t>(-expected_)))
+                throw ArgumentMismatch(single_name(), expected_, results_.size());
+            else
+                local_result = !callback_(results_);
         }
 
         if(local_result)
@@ -527,13 +532,13 @@ class Option : public OptionBase<Option> {
     /// @name Custom options
     ///@{
 
-    /// Set a custom option, typestring, expected, and changeable
-    void set_custom_option(std::string typeval, int expected = 1, bool changeable = false) {
+    /// Set a custom option, typestring, expected; locks changeable unless expected is -1
+    void set_custom_option(std::string typeval, int expected = 1) {
         typeval_ = typeval;
         expected_ = expected;
         if(expected == 0)
             required_ = false;
-        changeable_ = changeable;
+        changeable_ = expected < 0;
     }
 
     /// Set the default value string representation
diff --git a/tests/AppTest.cpp b/tests/AppTest.cpp
index e95ddf0a..4a06f0a7 100644
--- a/tests/AppTest.cpp
+++ b/tests/AppTest.cpp
@@ -119,7 +119,7 @@ TEST_F(TApp, DualOptions) {
     EXPECT_EQ(ans, vstr);
 
     args = {"--string=one", "--string=two"};
-    EXPECT_THROW(run(), CLI::ConversionError);
+    EXPECT_THROW(run(), CLI::ArgumentMismatch);
 }
 
 TEST_F(TApp, LotsOfFlags) {
@@ -737,7 +737,7 @@ TEST_F(TApp, FailSet) {
     app.add_set("-q,--quick", choice, {1, 2, 3});
 
     args = {"--quick", "3", "--quick=2"};
-    EXPECT_THROW(run(), CLI::ConversionError);
+    EXPECT_THROW(run(), CLI::ArgumentMismatch);
 
     app.reset();
 
@@ -770,7 +770,7 @@ TEST_F(TApp, InSetIgnoreCase) {
 
     app.reset();
     args = {"--quick=one", "--quick=two"};
-    EXPECT_THROW(run(), CLI::ConversionError);
+    EXPECT_THROW(run(), CLI::ArgumentMismatch);
 }
 
 TEST_F(TApp, VectorFixedString) {
@@ -1145,27 +1145,24 @@ TEST_F(TApp, CheckSubcomFail) {
     EXPECT_THROW(CLI::detail::AppFriend::parse_subcommand(&app, args), CLI::HorribleError);
 }
 
-// Added to test defaults on dual method
 TEST_F(TApp, OptionWithDefaults) {
     int someint = 2;
     app.add_option("-a", someint, "", true);
 
     args = {"-a1", "-a2"};
 
-    EXPECT_THROW(run(), CLI::ConversionError);
+    EXPECT_THROW(run(), CLI::ArgumentMismatch);
 }
 
-// Added to test defaults on dual method
 TEST_F(TApp, SetWithDefaults) {
     int someint = 2;
     app.add_set("-a", someint, {1, 2, 3, 4}, "", true);
 
     args = {"-a1", "-a2"};
 
-    EXPECT_THROW(run(), CLI::ConversionError);
+    EXPECT_THROW(run(), CLI::ArgumentMismatch);
 }
 
-// Added to test defaults on dual method
 TEST_F(TApp, SetWithDefaultsConversion) {
     int someint = 2;
     app.add_set("-a", someint, {1, 2, 3, 4}, "", true);
@@ -1175,14 +1172,13 @@ TEST_F(TApp, SetWithDefaultsConversion) {
     EXPECT_THROW(run(), CLI::ConversionError);
 }
 
-// Added to test defaults on dual method
 TEST_F(TApp, SetWithDefaultsIC) {
     std::string someint = "ho";
     app.add_set_ignore_case("-a", someint, {"Hi", "Ho"}, "", true);
 
     args = {"-aHi", "-aHo"};
 
-    EXPECT_THROW(run(), CLI::ConversionError);
+    EXPECT_THROW(run(), CLI::ArgumentMismatch);
 }
 
 // Added to test ->transform