diff --git a/tools/run_tests/run_csharp.sh b/tools/run_tests/run_csharp.sh
index 71cd87411f544eb65b753f065b873452ebb8548d..7ba21d460ef1fe6ac92537d577f6232cc1448c98 100755
--- a/tools/run_tests/run_csharp.sh
+++ b/tools/run_tests/run_csharp.sh
@@ -34,7 +34,7 @@ CONFIG=${CONFIG:-opt}
 NUNIT_CONSOLE="mono packages/NUnit.ConsoleRunner.3.2.0/tools/nunit3-console.exe"
 
 # nunit3-console fails if nunit.framework.dll is not found.
-cp -u src/csharp/packages/NUnit.3.2.0/lib/net45/nunit.framework.dll src/csharp/packages/NUnit.ConsoleRunner.3.2.0/tools/
+cp -n src/csharp/packages/NUnit.3.2.0/lib/net45/nunit.framework.dll src/csharp/packages/NUnit.ConsoleRunner.3.2.0/tools/
 
 # change to gRPC repo root
 cd $(dirname $0)/../..