diff --git a/lib/multi.c b/lib/multi.c
index 57c2ae8c0..d8de7e8e3 100644
--- a/lib/multi.c
+++ b/lib/multi.c
@@ -69,7 +69,7 @@
 #define CURL_MULTI_HANDLE 0x000bab1e
 
 #define GOOD_MULTI_HANDLE(x) \
-  ((x) && (x)->type == CURL_MULTI_HANDLE)
+  ((x) && (x)->magic == CURL_MULTI_HANDLE)
 
 static CURLMcode singlesocket(struct Curl_multi *multi,
                               struct Curl_easy *data);
@@ -360,7 +360,7 @@ struct Curl_multi *Curl_multi_handle(int hashsize, /* socket hash */
   if(!multi)
     return NULL;
 
-  multi->type = CURL_MULTI_HANDLE;
+  multi->magic = CURL_MULTI_HANDLE;
 
   if(Curl_mk_dnscache(&multi->hostcache))
     goto error;
@@ -2453,7 +2453,7 @@ CURLMcode curl_multi_cleanup(struct Curl_multi *multi)
     if(multi->in_callback)
       return CURLM_RECURSIVE_API_CALL;
 
-    multi->type = 0; /* not good anymore */
+    multi->magic = 0; /* not good anymore */
 
     /* Firsrt remove all remaining easy handles */
     data = multi->easyp;
diff --git a/lib/multihandle.h b/lib/multihandle.h
index aa9ca4bcd..f28c5899b 100644
--- a/lib/multihandle.h
+++ b/lib/multihandle.h
@@ -7,7 +7,7 @@
  *                            | (__| |_| |  _ <| |___
  *                             \___|\___/|_| \_\_____|
  *
- * Copyright (C) 1998 - 2020, Daniel Stenberg, <daniel@haxx.se>, et al.
+ * Copyright (C) 1998 - 2021, Daniel Stenberg, <daniel@haxx.se>, et al.
  *
  * This software is licensed as described in the file COPYING, which
  * you should have received as part of this distribution. The terms
@@ -83,7 +83,7 @@ typedef enum {
 struct Curl_multi {
   /* First a simple identifier to easier detect if a user mix up
      this multi handle with an easy handle. Set this to CURL_MULTI_HANDLE. */
-  long type;
+  unsigned int magic;
 
   /* We have a doubly-linked list with easy handles */
   struct Curl_easy *easyp;
diff --git a/lib/setopt.c b/lib/setopt.c
index 731ffeb36..409234fcd 100644
--- a/lib/setopt.c
+++ b/lib/setopt.c
@@ -2169,8 +2169,9 @@ CURLcode Curl_vsetopt(struct Curl_easy *data, CURLoption option, va_list param)
       data->share = NULL;
     }
 
-    /* use new share if it set */
-    data->share = set;
+    if(GOOD_SHARE_HANDLE(set))
+      /* use new share if it set */
+      data->share = set;
     if(data->share) {
 
       Curl_share_lock(data, CURL_LOCK_DATA_SHARE, CURL_LOCK_ACCESS_SINGLE);
diff --git a/lib/share.c b/lib/share.c
index 5ce983033..4f1804dbd 100644
--- a/lib/share.c
+++ b/lib/share.c
@@ -5,7 +5,7 @@
  *                            | (__| |_| |  _ <| |___
  *                             \___|\___/|_| \_\_____|
  *
- * Copyright (C) 1998 - 2020, Daniel Stenberg, <daniel@haxx.se>, et al.
+ * Copyright (C) 1998 - 2021, Daniel Stenberg, <daniel@haxx.se>, et al.
  *
  * This software is licensed as described in the file COPYING, which
  * you should have received as part of this distribution. The terms
@@ -37,6 +37,7 @@ curl_share_init(void)
 {
   struct Curl_share *share = calloc(1, sizeof(struct Curl_share));
   if(share) {
+    share->magic = CURL_GOOD_SHARE;
     share->specifier |= (1<<CURL_LOCK_DATA_SHARE);
 
     if(Curl_mk_dnscache(&share->hostcache)) {
@@ -59,6 +60,9 @@ curl_share_setopt(struct Curl_share *share, CURLSHoption option, ...)
   void *ptr;
   CURLSHcode res = CURLSHE_OK;
 
+  if(!GOOD_SHARE_HANDLE(share))
+    return CURLSHE_INVALID;
+
   if(share->dirty)
     /* don't allow setting options while one or more handles are already
        using this share */
@@ -184,7 +188,7 @@ curl_share_setopt(struct Curl_share *share, CURLSHoption option, ...)
 CURLSHcode
 curl_share_cleanup(struct Curl_share *share)
 {
-  if(share == NULL)
+  if(!GOOD_SHARE_HANDLE(share))
     return CURLSHE_INVALID;
 
   if(share->lockfunc)
@@ -218,6 +222,7 @@ curl_share_cleanup(struct Curl_share *share)
 
   if(share->unlockfunc)
     share->unlockfunc(NULL, CURL_LOCK_DATA_SHARE, share->clientdata);
+  share->magic = 0;
   free(share);
 
   return CURLSHE_OK;
diff --git a/lib/share.h b/lib/share.h
index 01aa9cda5..222e34ba6 100644
--- a/lib/share.h
+++ b/lib/share.h
@@ -7,7 +7,7 @@
  *                            | (__| |_| |  _ <| |___
  *                             \___|\___/|_| \_\_____|
  *
- * Copyright (C) 1998 - 2020, Daniel Stenberg, <daniel@haxx.se>, et al.
+ * Copyright (C) 1998 - 2021, Daniel Stenberg, <daniel@haxx.se>, et al.
  *
  * This software is licensed as described in the file COPYING, which
  * you should have received as part of this distribution. The terms
@@ -37,8 +37,12 @@
 #define CURL_VOLATILE volatile
 #endif
 
+#define CURL_GOOD_SHARE 0x7e117a1e
+#define GOOD_SHARE_HANDLE(x) ((x) && (x)->magic == CURL_GOOD_SHARE)
+
 /* this struct is libcurl-private, don't export details */
 struct Curl_share {
+  unsigned int magic; /* CURL_GOOD_SHARE */
   unsigned int specifier;
   CURL_VOLATILE unsigned int dirty;
 
diff --git a/lib/urldata.h b/lib/urldata.h
index 072caf002..1aa585077 100644
--- a/lib/urldata.h
+++ b/lib/urldata.h
@@ -1884,6 +1884,10 @@ struct Names {
  */
 
 struct Curl_easy {
+  /* First a simple identifier to easier detect if a user mix up this easy
+     handle with a multi handle. Set this to CURLEASY_MAGIC_NUMBER */
+  unsigned int magic;
+
   /* first, two fields for the linked list of these */
   struct Curl_easy *next;
   struct Curl_easy *prev;
@@ -1947,7 +1951,6 @@ struct Curl_easy {
 #ifdef USE_HYPER
   struct hyptransfer hyp;
 #endif
-  unsigned int magic;          /* set to a CURLEASY_MAGIC_NUMBER */
 };
 
 #define LIBCURL_NAME "libcurl"