diff --git a/src/ir/expressions/UnaryBooleanFunctionExpression.h b/src/ir/expressions/UnaryBooleanFunctionExpression.h
index 957c223a8..a59658040 100644
--- a/src/ir/expressions/UnaryBooleanFunctionExpression.h
+++ b/src/ir/expressions/UnaryBooleanFunctionExpression.h
@@ -39,9 +39,7 @@ namespace storm {
                  *
                  * @param The operator that is associated with this node.
                  */
-                FunctionType getFunctionType() const {
-                    return functionType;
-                }
+                FunctionType getFunctionType() const;
                 
                 virtual bool getValueAsBool(std::pair<std::vector<bool>, std::vector<int_fast64_t>> const* variableValues) const override;
                 
diff --git a/src/ir/expressions/UnaryNumericalFunctionExpression.h b/src/ir/expressions/UnaryNumericalFunctionExpression.h
index 3a1bfcab0..b5fda8e59 100644
--- a/src/ir/expressions/UnaryNumericalFunctionExpression.h
+++ b/src/ir/expressions/UnaryNumericalFunctionExpression.h
@@ -43,9 +43,7 @@ namespace storm {
                  *
                  * @param The operator that is associated with this node.
                  */
-                FunctionType getFunctionType() const {
-                    return functionType;
-                }
+                FunctionType getFunctionType() const;
                 
                 virtual void accept(ExpressionVisitor* visitor) override;