diff --git a/library/ctr_drbg.c b/library/ctr_drbg.c
index aefddfa..646196e 100644
--- a/library/ctr_drbg.c
+++ b/library/ctr_drbg.c
@@ -290,7 +290,8 @@
     unsigned char seed[MBEDTLS_CTR_DRBG_MAX_SEED_INPUT];
     size_t seedlen = 0;
 
-    if( ctx->entropy_len + len > MBEDTLS_CTR_DRBG_MAX_SEED_INPUT )
+    if( ctx->entropy_len > MBEDTLS_CTR_DRBG_MAX_SEED_INPUT ||
+        len > MBEDTLS_CTR_DRBG_MAX_SEED_INPUT - ctx->entropy_len )
         return( MBEDTLS_ERR_CTR_DRBG_INPUT_TOO_BIG );
 
     memset( seed, 0, MBEDTLS_CTR_DRBG_MAX_SEED_INPUT );