diff --git a/dtool/LocalSetup.pp b/dtool/LocalSetup.pp
index f029248cf2..e2e1650020 100644
--- a/dtool/LocalSetup.pp
+++ b/dtool/LocalSetup.pp
@@ -499,7 +499,7 @@ $[cdefine GLOBAL_OPERATOR_NEW_EXCEPTIONS]
 #define USE_MEMORY_MALLOC
 #define USE_MEMORY_NOWRAPPERS
 #if $[ALTERNATIVE_MALLOC]
-  #if $[HAVE_THREADS]
+  #if $[and $[HAVE_THREADS], $[not $[SIMPLE_THREADS]]]
     // A fast thread-safe alternative implementation.
     #set USE_MEMORY_PTMALLOC2 1
   #else
diff --git a/dtool/src/dtoolbase/dtoolbase.cxx b/dtool/src/dtoolbase/dtoolbase.cxx
index f4d6385695..8c7c7e7eab 100644
--- a/dtool/src/dtoolbase/dtoolbase.cxx
+++ b/dtool/src/dtoolbase/dtoolbase.cxx
@@ -35,7 +35,7 @@ bool __tau_shutdown = false;
 
 #if defined(USE_MEMORY_DLMALLOC)
 
-#ifdef HAVE_THREADS
+#if defined(HAVE_THREADS) && !defined(SIMPLE_THREADS)
 #error Cannot use dlmalloc library with threading enabled!
 #endif