diff --git a/src/sds.c b/src/sds.c
index f1e4f94..aeb9bc5 100644
--- a/src/sds.c
+++ b/src/sds.c
@@ -88,6 +88,7 @@ sds sdsnewlen(const void *init, size_t initlen) {
     int hdrlen = sdsHdrSize(type);
     unsigned char *fp; /* flags pointer. */
 
+    assert(initlen + hdrlen + 1 > initlen); /* Catch size_t overflow */
     sh = s_malloc(hdrlen+initlen+1);
     if (!init)
         memset(sh, 0, hdrlen+initlen+1);
@@ -204,6 +205,7 @@ sds sdsMakeRoomFor(sds s, size_t addlen) {
     len = sdslen(s);
     sh = (char*)s-sdsHdrSize(oldtype);
     newlen = (len+addlen);
+    assert(newlen > len);   /* Catch size_t overflow */
     if (newlen < SDS_MAX_PREALLOC)
         newlen *= 2;
     else
@@ -217,6 +219,7 @@ sds sdsMakeRoomFor(sds s, size_t addlen) {
     if (type == SDS_TYPE_5) type = SDS_TYPE_8;
 
     hdrlen = sdsHdrSize(type);
+    assert(hdrlen + newlen + 1 > len);  /* Catch size_t overflow */
     if (oldtype==type) {
         newsh = s_realloc(sh, hdrlen+newlen+1);
         if (newsh == NULL) return NULL;
diff --git a/src/zmalloc.c b/src/zmalloc.c
index 4dbef83..dca90c2 100644
--- a/src/zmalloc.c
+++ b/src/zmalloc.c
@@ -55,6 +55,12 @@ void zlibc_free(void *ptr) {
 #endif
 #endif
 
+#if PREFIX_SIZE > 0
+#define ASSERT_NO_SIZE_OVERFLOW(sz) assert((sz) + PREFIX_SIZE > (sz))
+#else
+#define ASSERT_NO_SIZE_OVERFLOW(sz)
+#endif
+
 /* Explicitly override malloc/free etc when using tcmalloc. */
 #if defined(USE_TCMALLOC)
 #define malloc(size) tc_malloc(size)
@@ -102,6 +108,7 @@ static void zmalloc_default_oom(size_t size) {
 static void (*zmalloc_oom_handler)(size_t) = zmalloc_default_oom;
 
 void *zmalloc(size_t size) {
+    ASSERT_NO_SIZE_OVERFLOW(size);
     void *ptr = malloc(size+PREFIX_SIZE);
 
     if (!ptr) zmalloc_oom_handler(size);
@@ -116,6 +123,7 @@ void *zmalloc(size_t size) {
 }
 
 void *zcalloc(size_t size) {
+    ASSERT_NO_SIZE_OVERFLOW(size);
     void *ptr = calloc(1, size+PREFIX_SIZE);
 
     if (!ptr) zmalloc_oom_handler(size);
@@ -130,6 +138,7 @@ void *zcalloc(size_t size) {
 }
 
 void *zrealloc(void *ptr, size_t size) {
+    ASSERT_NO_SIZE_OVERFLOW(size);
 #ifndef HAVE_MALLOC_SIZE
     void *realptr;
 #endif