diff --git a/bitcoin/signature.c b/bitcoin/signature.c
index 4de412dea..3f2dc18da 100644
--- a/bitcoin/signature.c
+++ b/bitcoin/signature.c
@@ -197,7 +197,7 @@ static bool IsValidSignatureEncoding(const unsigned char sig[], size_t len)
 
     // Verify that the length of the signature matches the sum of the length
     // of the elements.
-    if ((size_t)(lenR + lenS + 7) != len) return false;
+    if ((size_t)lenR + (size_t)lenS + 7 != len) return false;
 
     // Check whether the R element is an integer.
     if (sig[2] != 0x02) return false;