diff --git a/build-support/factor.sh b/build-support/factor.sh
index d8b547d8d6..9da4ae295a 100755
--- a/build-support/factor.sh
+++ b/build-support/factor.sh
@@ -195,8 +195,9 @@ find_architecture() {
 }
 
 write_test_program() {
-    $ECHO "#include <stdio.h>" > $C_WORD.c
-    $ECHO "int main(){printf(\"%ld\", (long)(8*sizeof(void*))); return 0; }" >> $C_WORD.c
+    #! Must be 'echo'
+    echo "#include <stdio.h>" > $C_WORD.c
+    echo "int main(){printf(\"%ld\", (long)(8*sizeof(void*))); return 0; }" >> $C_WORD.c
 }
 
 c_find_word_size() {