diff --git a/benchexec/tools/proton.py b/benchexec/tools/proton.py
index 27955fb09..c62308e0f 100644
--- a/benchexec/tools/proton.py
+++ b/benchexec/tools/proton.py
@@ -47,7 +47,7 @@ def determine_result(self, run):
             elif "FALSE(termination)" in result_str:
                 status = result.RESULT_FALSE_TERMINATION
 
-            elif "UNKNOWN" in output:
+            elif "UNKNOWN" in result_str:
                 status = result.RESULT_UNKNOWN
 
             elif "INTERNAL-ERROR" in result_str:
@@ -59,7 +59,4 @@ def determine_result(self, run):
         elif run.exit_code.value == 64 and "Usage error!" in output:
             status = "INVALID ARGUMENTS"
 
-        else:
-            status = result.RESULT_ERROR
-
         return status