diff --git a/dtool/src/prc/encryptStream.cxx b/dtool/src/prc/encryptStream.cxx
index 380296dd8c..90e9e33858 100644
--- a/dtool/src/prc/encryptStream.cxx
+++ b/dtool/src/prc/encryptStream.cxx
@@ -22,7 +22,7 @@
  */
 bool IDecryptStream::
 read_magic(const char *magic, size_t size) {
-  char this_magic[size];
+  char *this_magic = (char *)alloca(size);
   read(this_magic, size);
 
   if (!fail() && gcount() == size && memcmp(this_magic, magic, size) == 0) {