diff --git a/sshbn.c b/sshbn.c
index 1c5c46a1..a5e0552f 100644
--- a/sshbn.c
+++ b/sshbn.c
@@ -607,6 +607,7 @@ static void internal_add_shifted(BignumInt *number,
     addend = (BignumDblInt)n << bshift;
 
     while (addend) {
+        assert(word <= number[0]);
 	addend += number[word];
 	number[word] = (BignumInt) addend & BIGNUM_INT_MASK;
 	addend >>= BIGNUM_INT_BITS;