diff --git a/nvm.sh b/nvm.sh
index 70f90da..7feee24 100755
--- a/nvm.sh
+++ b/nvm.sh
@@ -206,7 +206,7 @@ nvm() {
         echo 'NVM Needs curl to proceed.' >&2;
       fi
 
-      if [ -z "`which shasum`" ]; then
+      if [ ! `\which shasum` ]; then
         shasum='sha1sum'
       fi