diff --git a/src/strings.sol b/src/strings.sol
index b449645a..075e17b6 100644
--- a/src/strings.sol
+++ b/src/strings.sol
@@ -34,7 +34,7 @@
  *      corresponding to the left and right parts of the string.
  */
 
-pragma solidity ^0.4.14;
+pragma solidity >0.4.13 <0.7.0;
 
 library strings {
     struct slice {
@@ -83,23 +83,23 @@ library strings {
         uint ret;
         if (self == 0)
             return 0;
-        if (self & 0xffffffffffffffffffffffffffffffff == 0) {
+        if (uint(self) & 0xffffffffffffffffffffffffffffffff == 0) {
             ret += 16;
             self = bytes32(uint(self) / 0x100000000000000000000000000000000);
         }
-        if (self & 0xffffffffffffffff == 0) {
+        if (uint(self) & 0xffffffffffffffff == 0) {
             ret += 8;
             self = bytes32(uint(self) / 0x10000000000000000);
         }
-        if (self & 0xffffffff == 0) {
+        if (uint(self) & 0xffffffff == 0) {
             ret += 4;
             self = bytes32(uint(self) / 0x100000000);
         }
-        if (self & 0xffff == 0) {
+        if (uint(self) & 0xffff == 0) {
             ret += 2;
             self = bytes32(uint(self) / 0x10000);
         }
-        if (self & 0xff == 0) {
+        if (uint(self) & 0xff == 0) {
             ret += 1;
         }
         return 32 - ret;
@@ -702,7 +702,7 @@ library strings {
         uint retptr;
         assembly { retptr := add(ret, 32) }
 
-        for(i = 0; i < parts.length; i++) {
+        for(uint i = 0; i < parts.length; i++) {
             memcpy(retptr, parts[i]._ptr, parts[i]._len);
             retptr += parts[i]._len;
             if (i < parts.length - 1) {