diff --git a/tests/run_tests.py b/tests/run_tests.py
index 40ab8c91b6..9d686b68a6 100755
--- a/tests/run_tests.py
+++ b/tests/run_tests.py
@@ -138,6 +138,7 @@ if len(sys.argv) > 1:
                     try:
                         tests__.index(j)
                     except ValueError:
+                        tests__.append(j)
 
         # Test name specified on command line
         else: