diff --git a/pb_encode.c b/pb_encode.c
index 409fec3..1a94202 100644
--- a/pb_encode.c
+++ b/pb_encode.c
@@ -264,7 +264,22 @@
         }
 
         /* Rest is proto3 singular fields */
-        if (PB_LTYPE(type) == PB_LTYPE_BYTES)
+        if (PB_LTYPE(type) <= PB_LTYPE_LAST_PACKABLE)
+        {
+            /* Simple integer / float fields */
+            pb_size_t i;
+            const char *p = (const char*)field->pData;
+            for (i = 0; i < field->data_size; i++)
+            {
+                if (p[i] != 0)
+                {
+                    return false;
+                }
+            }
+
+            return true;
+        }
+        else if (PB_LTYPE(type) == PB_LTYPE_BYTES)
         {
             const pb_bytes_array_t *bytes = (const pb_bytes_array_t*)field->pData;
             return bytes->size == 0;
@@ -302,27 +317,29 @@
             return true;
         }
     }
-    
+    else if (PB_ATYPE(type) == PB_ATYPE_POINTER)
     {
-        /* Catch-all branch that does byte-per-byte comparison for zero value.
-         *
-         * This is for all pointer fields, and for static PB_LTYPE_VARINT,
-         * UVARINT, SVARINT, FIXED32, FIXED64, EXTENSION fields, and also
-         * callback fields. These all have integer or pointer value which
-         * can be compared with 0.
-         */
-        pb_size_t i;
-        const char *p = (const char*)field->pData;
-        for (i = 0; i < field->data_size; i++)
-        {
-            if (p[i] != 0)
-            {
-                return false;
-            }
-        }
-
-        return true;
+        return field->pData == NULL;
     }
+    else if (PB_ATYPE(type) == PB_ATYPE_CALLBACK)
+    {
+        if (PB_LTYPE(type) == PB_LTYPE_EXTENSION)
+        {
+            const pb_extension_t *extension = *(const pb_extension_t* const *)field->pData;
+            return extension == NULL;
+        }
+        else if (field->descriptor->field_callback == pb_default_field_callback)
+        {
+            pb_callback_t *pCallback = (pb_callback_t*)field->pData;
+            return pCallback->funcs.encode == NULL;
+        }
+        else
+        {
+            return field->descriptor->field_callback == NULL;
+        }
+    }
+
+    return false; /* Not typically reached, safe default for weird special cases. */
 }
 
 /* Encode a field with static or pointer allocation, i.e. one whose data
diff --git a/tests/regression/issue_504/test.c b/tests/regression/issue_504/test.c
index 1ca86db..6c6693a 100644
--- a/tests/regression/issue_504/test.c
+++ b/tests/regression/issue_504/test.c
@@ -13,34 +13,65 @@
     int status = 0;
     uint8_t buffer[512] = {0};
     int i;
-    pb_ostream_t ostream;
-    MyMessage msg = MyMessage_init_zero;
-    char *pStr, *pStrAligned;
-    ostream = pb_ostream_from_buffer(buffer, sizeof(buffer));
 
-    /* copy STR to a malloced 0x100 aligned address */
-    pStr = malloc(sizeof(STR) + ALIGN);
-    pStrAligned = (char*)((uintptr_t)(pStr + ALIGN) & ~(ALIGN - 1));
-    memcpy(pStrAligned, STR, sizeof(STR));
+    {
+        pb_ostream_t ostream;
+        MyMessage msg = MyMessage_init_zero;
+        char *pStr, *pStrAligned;
 
-    msg.submessage.somestring = pStrAligned;
-    printf("%p: '%s'\n", msg.submessage.somestring, msg.submessage.somestring);
+        COMMENT("Test for false negatives with pointer value low byte 0x00")
+        ostream = pb_ostream_from_buffer(buffer, sizeof(buffer));
 
-    if (!pb_encode(&ostream, MyMessage_fields, &msg)) {
-        fprintf(stderr, "Encode failed: %s\n", PB_GET_ERROR(&ostream));
-        return 1;
+        /* copy STR to a malloced 0x100 aligned address */
+        pStr = malloc(sizeof(STR) + ALIGN);
+        pStrAligned = (char*)((uintptr_t)(pStr + ALIGN) & ~(ALIGN - 1));
+        memcpy(pStrAligned, STR, sizeof(STR));
+
+        msg.submessage.somestring = pStrAligned;
+        printf("%p: '%s'\n", msg.submessage.somestring, msg.submessage.somestring);
+
+        if (!pb_encode(&ostream, MyMessage_fields, &msg)) {
+            fprintf(stderr, "Encode failed: %s\n", PB_GET_ERROR(&ostream));
+            return 1;
+        }
+
+        free(pStr);
+        msg.submessage.somestring = NULL;
+
+        printf("response payload (%d):", (int)ostream.bytes_written);
+        for (i = 0; i < ostream.bytes_written; i++) {
+            printf("%02X", buffer[i]);
+        }
+        printf("\n");
+
+        TEST(ostream.bytes_written != 0);
     }
 
-    free(pStr);
-    msg.submessage.somestring = NULL;
+    {
+        pb_ostream_t ostream;
+        struct {
+            MyMessage msg;
+            uint32_t bar;
+         } msg = {MyMessage_init_zero, 0};
 
-    printf("response payload (%d):", (int)ostream.bytes_written);
-    for (i = 0; i < ostream.bytes_written; i++) {
-        printf("%02X", buffer[i]);
+        COMMENT("Test for false positives with data after end of struct")
+        ostream = pb_ostream_from_buffer(buffer, sizeof(buffer));
+
+        msg.bar = 0xFFFFFFFF;
+
+        if (!pb_encode(&ostream, MyMessage_fields, &msg)) {
+            fprintf(stderr, "Encode failed: %s\n", PB_GET_ERROR(&ostream));
+            return 1;
+        }
+
+        printf("response payload (%d):", (int)ostream.bytes_written);
+        for (i = 0; i < ostream.bytes_written; i++) {
+            printf("%02X", buffer[i]);
+        }
+        printf("\n");
+
+        TEST(ostream.bytes_written == 0);
     }
-    printf("\n");
-
-    TEST(ostream.bytes_written != 0);
 
     return status;
 }
diff --git a/tests/regression/issue_504/test.proto b/tests/regression/issue_504/test.proto
index bdafd3c..7a6f0d4 100644
--- a/tests/regression/issue_504/test.proto
+++ b/tests/regression/issue_504/test.proto
@@ -10,4 +10,10 @@
 message SubMessage
 {
     string somestring = 1 [(nanopb).type = FT_POINTER];
+    SubMessage2 submsg2 = 2 [(nanopb).type = FT_POINTER];
 }
+
+message SubMessage2
+{
+    uint32 foo = 1;
+}
\ No newline at end of file