diff --git a/src/uint256.cpp b/src/uint256.cpp
--- a/src/uint256.cpp
+++ b/src/uint256.cpp
@@ -32,18 +32,17 @@
     }
 
     // hex string to uint
-    const char *pbegin = psz;
-    while (::HexDigit(*psz) != -1) {
-        psz++;
+    size_t digits = 0;
+    while (::HexDigit(psz[digits]) != -1) {
+        digits++;
     }
 
-    psz--;
     uint8_t *p1 = (uint8_t *)data;
     uint8_t *pend = p1 + WIDTH;
-    while (psz >= pbegin && p1 < pend) {
-        *p1 = ::HexDigit(*psz--);
-        if (psz >= pbegin) {
-            *p1 |= uint8_t(::HexDigit(*psz--) << 4);
+    while (digits > 0 && p1 < pend) {
+        *p1 = ::HexDigit(psz[--digits]);
+        if (digits > 0) {
+            *p1 |= uint8_t(::HexDigit(psz[--digits])) << 4;
             p1++;
         }
     }