diff --git a/lib/legacy/zstd_legacy.h b/lib/legacy/zstd_legacy.h
index 56b3e82..ab9634b 100644
--- a/lib/legacy/zstd_legacy.h
+++ b/lib/legacy/zstd_legacy.h
@@ -54,8 +54,11 @@
     @return : > 0 if supported by legacy decoder. 0 otherwise.
               return value is the version.
 */
-MEM_STATIC unsigned ZSTD_isLegacy (U32 magicNumberLE)
+MEM_STATIC unsigned ZSTD_isLegacy(const void* src, size_t srcSize)
 {
+    U32 magicNumberLE;
+    if (srcSize<4) return 0;
+    magicNumberLE = MEM_readLE32(src);
     switch(magicNumberLE)
     {
         case ZSTDv01_magicNumberLE:return 1;
@@ -73,10 +76,8 @@
 {
     if (srcSize < 4) return 0;
 
-    {   U32 const magic = MEM_readLE32(src);
-        U32 const version = ZSTD_isLegacy(magic);
-        if (!version) return 0;   /* not a supported legacy format */
-        if (version < 5) return 0;  /* no decompressed size in frame header */
+    {   U32 const version = ZSTD_isLegacy(src, srcSize);
+        if (version < 5) return 0;  /* no decompressed size in frame header, or not a legacy format */
         if (version==5) {
             ZSTDv05_parameters fParams;
             size_t const frResult = ZSTDv05_getFrameParams(&fParams, src, srcSize);
@@ -96,20 +97,20 @@
 MEM_STATIC size_t ZSTD_decompressLegacy(
                      void* dst, size_t dstCapacity,
                const void* src, size_t compressedSize,
-               const void* dict,size_t dictSize,
-                     U32 magicNumberLE)
+               const void* dict,size_t dictSize)
 {
-    switch(magicNumberLE)
+    U32 const version = ZSTD_isLegacy(src, compressedSize);
+    switch(version)
     {
-        case ZSTDv01_magicNumberLE :
+        case 1 :
             return ZSTDv01_decompress(dst, dstCapacity, src, compressedSize);
-        case ZSTDv02_magicNumber :
+        case 2 :
             return ZSTDv02_decompress(dst, dstCapacity, src, compressedSize);
-        case ZSTDv03_magicNumber :
+        case 3 :
             return ZSTDv03_decompress(dst, dstCapacity, src, compressedSize);
-        case ZSTDv04_magicNumber :
+        case 4 :
             return ZSTDv04_decompress(dst, dstCapacity, src, compressedSize);
-        case ZSTDv05_MAGICNUMBER :
+        case 5 :
             {   size_t result;
                 ZSTDv05_DCtx* const zd = ZSTDv05_createDCtx();
                 if (zd==NULL) return ERROR(memory_allocation);
@@ -117,7 +118,7 @@
                 ZSTDv05_freeDCtx(zd);
                 return result;
             }
-        case ZSTDv06_MAGICNUMBER :
+        case 6 :
             {   size_t result;
                 ZSTDv06_DCtx* const zd = ZSTDv06_createDCtx();
                 if (zd==NULL) return ERROR(memory_allocation);