diff --git a/autoninja b/autoninja
index 4c10c48e6e..b9990ee8aa 100755
--- a/autoninja
+++ b/autoninja
@@ -5,7 +5,7 @@
 # found in the LICENSE file.
 
 # In git bash on Windows, invoke the batch file.
-if [ "$(expr substr $(uname -s) 1 10)" == "MINGW64_NT" ]; then
+if [ "$(expr "$(uname -s)" : "^MINGW64_NT")" == "10" ]; then
     autoninja.bat "$@"
     exit
 fi