diff --git a/libsolidity/analysis/TypeChecker.cpp b/libsolidity/analysis/TypeChecker.cpp
index a63b3f27d..8ec9f649d 100644
--- a/libsolidity/analysis/TypeChecker.cpp
+++ b/libsolidity/analysis/TypeChecker.cpp
@@ -1849,7 +1849,8 @@ void TypeChecker::endVisit(BinaryOperation const& _operation)
 	if (userDefinedOperatorResult)
 		solAssert(
 			userDefinedFunctionType->returnParameterTypes().size() == 1 &&
-			*userDefinedFunctionType->returnParameterTypes().front() == *_operation.annotation().type
+			*userDefinedFunctionType->returnParameterTypes().front() == *_operation.annotation().type,
+			"User defined operator has an invalid return parameter."
 		);
 	else if (_operation.getOperator() == Token::Exp || _operation.getOperator() == Token::SHL)
 	{
diff --git a/libsolidity/ast/Types.cpp b/libsolidity/ast/Types.cpp
index 13e6b8028..4841240c0 100644
--- a/libsolidity/ast/Types.cpp
+++ b/libsolidity/ast/Types.cpp
@@ -404,10 +404,19 @@ Result<FunctionDefinition const*> Type::userDefinedOperator(Token _token, ASTNod
 				function.libraryFunction() ? function.typeViaContractName() : function.type()
 			);
 			solAssert(functionType && !functionType->parameterTypes().empty());
-			solAssert(isImplicitlyConvertibleTo(*functionType->parameterTypes().front()));
+
+			Type const* expectedReturnType =
+				TokenTraits::isCompareOp(_token) ?
+				TypeProvider::boolean() : functionType->parameterTypes().front();
+
 			if (
-				(_unaryOperation && function.parameterList().parameters().size() == 1) ||
-				(!_unaryOperation && function.parameterList().parameters().size() == 2)
+				isImplicitlyConvertibleTo(*functionType->parameterTypes().front()) &&
+				function.returnParameterList()->parameters().size() == 1 &&
+				function.returnParameterList()->parameters().front()->type()->isImplicitlyConvertibleTo(*expectedReturnType) &&
+				(
+					(_unaryOperation && function.parameterList().parameters().size() == 1) ||
+					(!_unaryOperation && function.parameterList().parameters().size() == 2)
+				)
 			)
 				seenFunctions.insert(&function);
 		}
diff --git a/test/libsolidity/syntaxTests/operators/custom/operator_invalid_parameters.sol b/test/libsolidity/syntaxTests/operators/custom/operator_invalid_parameters.sol
new file mode 100644
index 000000000..1a84fc47a
--- /dev/null
+++ b/test/libsolidity/syntaxTests/operators/custom/operator_invalid_parameters.sol
@@ -0,0 +1,33 @@
+type Int is int256;
+
+using {
+    add as +, sub as -, div as /
+} for Int;
+
+function add(Int) pure returns (Int) {
+    return Int.wrap(0);
+}
+
+function sub(Int, Int, Int) pure returns (Int) {
+    return Int.wrap(1);
+}
+
+function div(int256, int256) pure returns (Int) {
+    return Int.wrap(2);
+}
+
+function f() pure {
+    Int.wrap(0) + Int.wrap(1);
+    Int.wrap(0) - Int.wrap(0);
+    Int.wrap(0) / Int.wrap(0);
+    Int.wrap(0) * Int.wrap(0);
+}
+
+// ----
+// TypeError 1884: (33-36): The function "add" needs to have two parameters of equal type to be used for the operator +.
+// TypeError 8112: (43-46): The function "sub" needs to have one or two parameters to be used for the operator -.
+// TypeError 3100: (53-56): The function "div" cannot be bound to the type "Int" because the type cannot be implicitly converted to the first argument of the function ("int256").
+// TypeError 2271: (317-342): Operator + not compatible with types Int and Int. No matching user-defined operator found.
+// TypeError 2271: (348-373): Operator - not compatible with types Int and Int. No matching user-defined operator found.
+// TypeError 2271: (379-404): Operator / not compatible with types Int and Int. No matching user-defined operator found.
+// TypeError 2271: (410-435): Operator * not compatible with types Int and Int. No matching user-defined operator found.
diff --git a/test/libsolidity/syntaxTests/operators/custom/operator_invalid_return_parameters.sol b/test/libsolidity/syntaxTests/operators/custom/operator_invalid_return_parameters.sol
new file mode 100644
index 000000000..7a97461da
--- /dev/null
+++ b/test/libsolidity/syntaxTests/operators/custom/operator_invalid_return_parameters.sol
@@ -0,0 +1,49 @@
+type Int is int256;
+
+using {
+    add as +,
+    div as /,
+    unsub as -,
+    gt as >,
+    lt as <
+} for Int;
+
+function add(Int x, Int y) pure returns (int256) {
+    return 0;
+}
+
+function div(Int x, Int y) pure {
+    x = y;
+}
+
+function unsub(Int) pure returns (Int, Int) {
+    return (Int.wrap(0), Int.wrap(1));
+}
+
+function gt(Int, Int) pure returns (Int) {
+    return Int.wrap(0);
+}
+
+function lt(Int, Int) pure returns (bool, Int) {
+    return (true, Int.wrap(1));
+}
+
+function f() pure {
+    Int.wrap(0) + Int.wrap(1);
+    Int.wrap(0) / Int.wrap(0);
+    -Int.wrap(0);
+    Int.wrap(0) < Int.wrap(0);
+    Int.wrap(0) > Int.wrap(0);
+}
+
+// ----
+// TypeError 7743: (33-36): The function "add" needs to return exactly one value of type Int to be used for the operator +.
+// TypeError 7743: (47-50): The function "div" needs to return exactly one value of type Int to be used for the operator /.
+// TypeError 7743: (61-66): The function "unsub" needs to return exactly one value of type Int to be used for the operator -.
+// TypeError 7995: (77-79): The function "gt" needs to return exactly one value of type bool to be used for the operator >.
+// TypeError 7995: (90-92): The function "lt" needs to return exactly one value of type bool to be used for the operator <.
+// TypeError 2271: (492-517): Operator + not compatible with types Int and Int. No matching user-defined operator found.
+// TypeError 2271: (523-548): Operator / not compatible with types Int and Int. No matching user-defined operator found.
+// TypeError 4907: (554-566): Unary operator - cannot be applied to type Int
+// TypeError 2271: (572-597): Operator < not compatible with types Int and Int. No matching user-defined operator found.
+// TypeError 2271: (603-628): Operator > not compatible with types Int and Int. No matching user-defined operator found.
diff --git a/test/libsolidity/syntaxTests/operators/custom/operator_wrong_number_params_return.sol b/test/libsolidity/syntaxTests/operators/custom/operator_wrong_number_params_return.sol
deleted file mode 100644
index 34b69a259..000000000
--- a/test/libsolidity/syntaxTests/operators/custom/operator_wrong_number_params_return.sol
+++ /dev/null
@@ -1,39 +0,0 @@
-type Int is int256;
-
-using {
-    add as +, sub as -,
-    mul as *, div as /,
-    gt as >, lt as <
-} for Int;
-
-function add(Int) pure returns (Int) {
-    return Int.wrap(0);
-}
-
-function sub(Int, Int, Int) pure returns (Int) {
-    return Int.wrap(1);
-}
-
-function mul(Int) pure returns (Int) {
-    return Int.wrap(2);
-}
-
-function div(Int x, Int y) pure {
-    x = y;
-}
-
-function gt(Int, Int) pure returns (Int) {
-    return Int.wrap(0);
-}
-
-function lt(Int, Int) pure returns (bool, Int) {
-    return (true, Int.wrap(1));
-}
-
-// ----
-// TypeError 1884: (33-36): The function "add" needs to have two parameters of equal type to be used for the operator +.
-// TypeError 8112: (43-46): The function "sub" needs to have one or two parameters to be used for the operator -.
-// TypeError 1884: (57-60): The function "mul" needs to have two parameters of equal type to be used for the operator *.
-// TypeError 7743: (67-70): The function "div" needs to return exactly one value of type Int to be used for the operator /.
-// TypeError 7995: (81-83): The function "gt" needs to return exactly one value of type bool to be used for the operator >.
-// TypeError 7995: (90-92): The function "lt" needs to return exactly one value of type bool to be used for the operator <.