diff --git a/src/bignum.h b/src/bignum.h
index 95e21977e7..daf5f6883a 100644
--- a/src/bignum.h
+++ b/src/bignum.h
@@ -301,7 +301,7 @@ 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 cfc2eb128e..0947816785 100644
--- a/src/uint256.h
+++ b/src/uint256.h
@@ -308,7 +308,7 @@ 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;
diff --git a/src/util.cpp b/src/util.cpp
index 1eda59bafe..a5427c061b 100644
--- a/src/util.cpp
+++ b/src/util.cpp
@@ -636,7 +636,7 @@ vector<unsigned char> DecodeBase64(const char* p, bool* pfInvalid)
 
     while (1)
     {
-         int dec = decode64_table[*p];
+         int dec = decode64_table[(unsigned char)*p];
          if (dec == -1) break;
          p++;
          switch (mode)
@@ -676,12 +676,12 @@ vector<unsigned char> DecodeBase64(const char* p, bool* pfInvalid)
                 break;
 
             case 2: // 4n+2 base64 characters processed: require '=='
-                if (left || p[0] != '=' || p[1] != '=' || decode64_table[p[2]] != -1)
+                if (left || p[0] != '=' || p[1] != '=' || decode64_table[(unsigned char)p[2]] != -1)
                     *pfInvalid = true;
                 break;
 
             case 3: // 4n+3 base64 characters processed: require '='
-                if (left || p[0] != '=' || decode64_table[p[1]] != -1)
+                if (left || p[0] != '=' || decode64_table[(unsigned char)p[1]] != -1)
                     *pfInvalid = true;
                 break;
         }