diff --git a/win32/win32.c b/win32/win32.c
index f19c097..d292425 100644
--- a/win32/win32.c
+++ b/win32/win32.c
@@ -1117,6 +1117,7 @@ CreateChild(const WCHAR *cmd, const WCHAR *prog, SECURITY_ATTRIBUTES *psa,
     PROCESS_INFORMATION aProcessInformation;
     SECURITY_ATTRIBUTES sa;
     struct ChildRecord *child;
+    WCHAR tmpCmd[MAX_PATH * 2];
 
     if (!cmd && !prog) {
 	errno = EFAULT;
@@ -1167,8 +1168,11 @@ CreateChild(const WCHAR *cmd, const WCHAR *prog, SECURITY_ATTRIBUTES *psa,
 	return NULL;
     }
 
+    /* Make a copy of cmd so CreateProcessW() can modify it if it needs to. */
+    _tcscpy_s(tmpCmd, MAX_PATH *2, (WCHAR *)cmd);
+
     RUBY_CRITICAL({
-	fRet = CreateProcessW(prog, (WCHAR *)cmd, psa, psa,
+	fRet = CreateProcessW(prog, tmpCmd, psa, psa,
 			      psa->bInheritHandle, dwCreationFlags, NULL, NULL,
 			      &aStartupInfo, &aProcessInformation);
 	errno = map_errno(GetLastError());