diff --git a/src/core/inet_chksum.c b/src/core/inet_chksum.c
index 646074c6..e027169e 100644
--- a/src/core/inet_chksum.c
+++ b/src/core/inet_chksum.c
@@ -315,10 +315,10 @@ inet_chksum_pseudo(struct pbuf *p, u8_t proto, u16_t proto_len,
 
   addr = ip4_addr_get_u32(src);
   acc = (addr & 0xffffUL);
-  acc += ((addr >> 16) & 0xffffUL);
+  acc = (u32_t)(acc + ((addr >> 16) & 0xffffUL));
   addr = ip4_addr_get_u32(dest);
-  acc += (addr & 0xffffUL);
-  acc += ((addr >> 16) & 0xffffUL);
+  acc = (u32_t)(acc + (addr & 0xffffUL));
+  acc = (u32_t)(acc + ((addr >> 16) & 0xffffUL));
   /* fold down to 16 bits */
   acc = FOLD_U32T(acc);
   acc = FOLD_U32T(acc);
@@ -349,11 +349,11 @@ ip6_chksum_pseudo(struct pbuf *p, u8_t proto, u16_t proto_len,
 
   for (addr_part = 0; addr_part < 4; addr_part++) {
     addr = src->addr[addr_part];
-    acc += (addr & 0xffffUL);
-    acc += ((addr >> 16) & 0xffffUL);
+    acc = (u32_t)(acc + (addr & 0xffffUL));
+    acc = (u32_t)(acc + ((addr >> 16) & 0xffffUL));
     addr = dest->addr[addr_part];
-    acc += (addr & 0xffffUL);
-    acc += ((addr >> 16) & 0xffffUL);
+    acc = (u32_t)(acc + (addr & 0xffffUL));
+    acc = (u32_t)(acc + ((addr >> 16) & 0xffffUL));
   }
   /* fold down to 16 bits */
   acc = FOLD_U32T(acc);
@@ -461,10 +461,10 @@ inet_chksum_pseudo_partial(struct pbuf *p, u8_t proto, u16_t proto_len,
 
   addr = ip4_addr_get_u32(src);
   acc = (addr & 0xffffUL);
-  acc += ((addr >> 16) & 0xffffUL);
+  acc = (u32_t)(acc + ((addr >> 16) & 0xffffUL));
   addr = ip4_addr_get_u32(dest);
-  acc += (addr & 0xffffUL);
-  acc += ((addr >> 16) & 0xffffUL);
+  acc = (u32_t)(acc + (addr & 0xffffUL));
+  acc = (u32_t)(acc + ((addr >> 16) & 0xffffUL));
   /* fold down to 16 bits */
   acc = FOLD_U32T(acc);
   acc = FOLD_U32T(acc);
@@ -497,11 +497,11 @@ ip6_chksum_pseudo_partial(struct pbuf *p, u8_t proto, u16_t proto_len,
 
   for (addr_part = 0; addr_part < 4; addr_part++) {
     addr = src->addr[addr_part];
-    acc += (addr & 0xffffUL);
-    acc += ((addr >> 16) & 0xffffUL);
+    acc = (u32_t)(acc + (addr & 0xffffUL));
+    acc = (u32_t)(acc + ((addr >> 16) & 0xffffUL));
     addr = dest->addr[addr_part];
-    acc += (addr & 0xffffUL);
-    acc += ((addr >> 16) & 0xffffUL);
+    acc = (u32_t)(acc + (addr & 0xffffUL));
+    acc = (u32_t)(acc + ((addr >> 16) & 0xffffUL));
   }
   /* fold down to 16 bits */
   acc = FOLD_U32T(acc);
diff --git a/src/include/lwip/inet_chksum.h b/src/include/lwip/inet_chksum.h
index 4e23d7f1..76893ef5 100644
--- a/src/include/lwip/inet_chksum.h
+++ b/src/include/lwip/inet_chksum.h
@@ -49,7 +49,7 @@
 
 /** Split an u32_t in two u16_ts and add them up */
 #ifndef FOLD_U32T
-#define FOLD_U32T(u)          (((u) >> 16) + ((u) & 0x0000ffffUL))
+#define FOLD_U32T(u)          ((u32_t)(((u) >> 16) + ((u) & 0x0000ffffUL)))
 #endif
 
 #if LWIP_CHECKSUM_ON_COPY