Index: generic/tclObj.c
==================================================================
--- generic/tclObj.c
+++ generic/tclObj.c
@@ -3087,16 +3087,20 @@
 
 		if (mp_to_unsigned_bin_n(&big, bytes, &numBytes) == MP_OKAY) {
 		    while (numBytes-- > 0) {
 			value = (value << CHAR_BIT) | *bytes++;
 		    }
-		    if (big.sign) {
-			*wideIntPtr = - (Tcl_WideInt) value;
+		    if ( (!big.sign) && ((Tcl_WideInt)value < 0) ) {
+			/* signed overflow - fall through to produce an error */
 		    } else {
-			*wideIntPtr = (Tcl_WideInt) value;
+			if (big.sign) {
+			    *wideIntPtr = - (Tcl_WideInt) value;
+			} else {
+			    *wideIntPtr = (Tcl_WideInt) value;
+			}
+			return TCL_OK;
 		    }
-		    return TCL_OK;
 		}
 	    }
 	    if (interp != NULL) {
 		const char *s = "integer value too large to represent";
 		Tcl_Obj* msg = Tcl_NewStringObj(s, -1);