diff --git a/src/bignum.h b/src/bignum.h
index 6e8d3cb8aba..641ebf4b051 100644
--- a/src/bignum.h
+++ b/src/bignum.h
@@ -300,7 +300,7 @@ public:
         while (isxdigit(*psz))
         {
             *this <<= 4;
-            int n = phexdigit[*psz++];
+            int n = phexdigit[(unsigned char)*psz++];
             *this += n;
         }
         if (fNegative)
diff --git a/src/uint256.h b/src/uint256.h
index ae263346a86..07809e49a8f 100644
--- a/src/uint256.h
+++ b/src/uint256.h
@@ -315,7 +315,7 @@ public:
         // hex string to uint
         static char phexdigit[256] = { 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,1,2,3,4,5,6,7,8,9,0,0,0,0,0,0, 0,0xa,0xb,0xc,0xd,0xe,0xf,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0xa,0xb,0xc,0xd,0xe,0xf,0,0,0,0,0,0,0,0,0 };
         const char* pbegin = psz;
-        while (phexdigit[*psz] || *psz == '0')
+        while (phexdigit[(unsigned char)*psz] || *psz == '0')
             psz++;
         psz--;
         unsigned char* p1 = (unsigned char*)pn;