diff --git a/lib/common/mem.h b/lib/common/mem.h
index 7a3f72141..3cacd216a 100644
--- a/lib/common/mem.h
+++ b/lib/common/mem.h
@@ -48,14 +48,15 @@ MEM_STATIC void MEM_check(void) { MEM_STATIC_ASSERT((sizeof(size_t)==4) || (size
 *****************************************************************/
 #if  !defined (__VMS) && (defined (__cplusplus) || (defined (__STDC_VERSION__) && (__STDC_VERSION__ >= 199901L) /* C99 */) )
 # include <stdint.h>
-  typedef  uint8_t BYTE;
-  typedef uint16_t U16;
-  typedef  int16_t S16;
-  typedef uint32_t U32;
-  typedef  int32_t S32;
-  typedef uint64_t U64;
-  typedef  int64_t S64;
-  typedef intptr_t iPtrDiff;
+  typedef   uint8_t BYTE;
+  typedef  uint16_t U16;
+  typedef   int16_t S16;
+  typedef  uint32_t U32;
+  typedef   int32_t S32;
+  typedef  uint64_t U64;
+  typedef   int64_t S64;
+  typedef  intptr_t iPtrDiff;
+  typedef uintptr_t uPtrDiff;
 #else
   typedef unsigned char      BYTE;
   typedef unsigned short      U16;
@@ -65,6 +66,7 @@ MEM_STATIC void MEM_check(void) { MEM_STATIC_ASSERT((sizeof(size_t)==4) || (size
   typedef unsigned long long  U64;
   typedef   signed long long  S64;
   typedef ptrdiff_t      iPtrDiff;
+  typedef size_t         uPtrDiff;
 #endif
 
 
diff --git a/lib/decompress/zstd_decompress.c b/lib/decompress/zstd_decompress.c
index 482c334ff..943bdf94e 100644
--- a/lib/decompress/zstd_decompress.c
+++ b/lib/decompress/zstd_decompress.c
@@ -876,7 +876,7 @@ typedef struct {
     size_t prevOffset[ZSTD_REP_NUM];
     const BYTE* base;
     size_t pos;
-    iPtrDiff gotoDict;
+    uPtrDiff gotoDict;
 } seqState_t;
 
 
@@ -1033,7 +1033,7 @@ size_t ZSTD_execSequence(BYTE* op,
     if (sequence.offset > (size_t)(oLitEnd - base)) {
         /* offset beyond prefix */
         if (sequence.offset > (size_t)(oLitEnd - vBase)) return ERROR(corruption_detected);
-        match += (dictEnd-base);
+        match = dictEnd + (match - base);
         if (match + sequence.matchLength <= dictEnd) {
             memmove(oLitEnd, match, sequence.matchLength);
             return sequenceLength;
@@ -1356,7 +1356,7 @@ static size_t ZSTD_decompressSequencesLong(
         { U32 i; for (i=0; i<ZSTD_REP_NUM; i++) seqState.prevOffset[i] = dctx->entropy.rep[i]; }
         seqState.base = base;
         seqState.pos = (size_t)(op-base);
-        seqState.gotoDict = (iPtrDiff)(dictEnd - base);
+        seqState.gotoDict = (uPtrDiff)dictEnd - (uPtrDiff)base; /* cast to avoid undefined behaviour */
         CHECK_E(BIT_initDStream(&seqState.DStream, ip, iend-ip), corruption_detected);
         FSE_initDState(&seqState.stateLL, &seqState.DStream, dctx->LLTptr);
         FSE_initDState(&seqState.stateOffb, &seqState.DStream, dctx->OFTptr);