diff --git a/reactos/subsystems/mvdm/ntvdm/dos/dos32krnl/memory.c b/reactos/subsystems/mvdm/ntvdm/dos/dos32krnl/memory.c
index 73a554d09c3..892bbbd3ffd 100644
--- a/reactos/subsystems/mvdm/ntvdm/dos/dos32krnl/memory.c
+++ b/reactos/subsystems/mvdm/ntvdm/dos/dos32krnl/memory.c
@@ -319,6 +319,7 @@ BOOLEAN DosFreeMemory(WORD BlockData)
     PDOS_MCB Mcb = SEGMENT_TO_MCB(BlockData - 1);
 
     DPRINT("DosFreeMemory: BlockData 0x%04X\n", BlockData);
+    if (BlockData == 0) return FALSE;
 
     /* Make sure the MCB is valid */
     if (Mcb->BlockType != 'M' && Mcb->BlockType != 'Z')