diff --git a/library/pkcs7.c b/library/pkcs7.c
index 36b49f5..0869c2e 100644
--- a/library/pkcs7.c
+++ b/library/pkcs7.c
@@ -316,7 +316,7 @@
         goto out;
     }
 
-    signer->issuer_raw.len =  *p - signer->issuer_raw.p;
+    signer->issuer_raw.len =  (size_t) (*p - signer->issuer_raw.p);
 
     ret = mbedtls_x509_get_serial(p, end_issuer_and_sn, &signer->serial);
     if (ret != 0) {