diff --git a/magick/blob.c b/magick/blob.c
index 0e30d90..98cd9fe 100644
--- a/magick/blob.c
+++ b/magick/blob.c
@@ -983,7 +983,7 @@
     }
   *length=(size_t) MagickMin((MagickSizeType) offset,extent);
   blob=(unsigned char *) NULL;
-  if (~(*length) >= MaxTextExtent)
+  if (~(*length) >= (MaxTextExtent-1))
     blob=(unsigned char *) AcquireQuantumMemory(*length+MaxTextExtent,
       sizeof(*blob));
   if (blob == (unsigned char *) NULL)