diff --git a/src/main/py/read_math_function.py b/src/main/py/read_math_function.py
index be726a5..a1d3868 100644
--- a/src/main/py/read_math_function.py
+++ b/src/main/py/read_math_function.py
@@ -75,4 +75,6 @@ def extract_constant(function):
 
 def simplify_exponential_function(type, function):
     if type != "e":
-        return False
\ No newline at end of file
+        return False
+    if 'x' not in function:
+        return "Der Ausdruck muss die Variable 'x' enthalten."
\ No newline at end of file