diff --git a/src/fileio.c b/src/fileio.c
index ea1f338..bcb8fef 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -3011,6 +3011,9 @@
 
 	    /* Remove cryptmethod specific header from the text. */
 	    header_len = crypt_get_header_len(method);
+	    if (*sizep <= header_len)
+		/* invalid header, buffer can't be encrypted */
+		return NULL;
 	    *filesizep += header_len;
 	    *sizep -= header_len;
 	    mch_memmove(ptr, ptr + header_len, (size_t)*sizep);