diff --git a/src/crypto/fips_prf_internal.c b/src/crypto/fips_prf_internal.c
index a85cb14d7..1e0c453a1 100644
--- a/src/crypto/fips_prf_internal.c
+++ b/src/crypto/fips_prf_internal.c
@@ -28,13 +28,14 @@ int fips186_2_prf(const u8 *seed, size_t seed_len, u8 *x, size_t xlen)
 	u8 *xpos = x;
 	u32 carry;
 
-	if (seed_len > sizeof(xkey))
+	if (seed_len < sizeof(xkey))
+		os_memset(xkey + seed_len, 0, sizeof(xkey) - seed_len);
+	else
 		seed_len = sizeof(xkey);
 
 	/* FIPS 186-2 + change notice 1 */
 
 	os_memcpy(xkey, seed, seed_len);
-	os_memset(xkey + seed_len, 0, 64 - seed_len);
 	t[0] = 0x67452301;
 	t[1] = 0xEFCDAB89;
 	t[2] = 0x98BADCFE;