diff --git a/objectivec/GPBCodedInputStream.m b/objectivec/GPBCodedInputStream.m
index acba715..2b578dd 100644
--- a/objectivec/GPBCodedInputStream.m
+++ b/objectivec/GPBCodedInputStream.m
@@ -227,7 +227,13 @@
   state->lastTag = ReadRawVarint32(state);
   if (state->lastTag == 0) {
     // If we actually read zero, that's not a valid tag.
-    RaiseException(GPBCodedInputStreamErrorInvalidTag, @"Last tag can't be 0");
+    RaiseException(GPBCodedInputStreamErrorInvalidTag,
+                   @"A zero tag on the wire is invalid.");
+  }
+  // Tags have to include a valid wireformat, check that also.
+  if (!GPBWireFormatIsValidTag(state->lastTag)) {
+    RaiseException(GPBCodedInputStreamErrorInvalidTag,
+                   @"Invalid wireformat in tag.");
   }
   return state->lastTag;
 }
@@ -352,6 +358,7 @@
 }
 
 - (BOOL)skipField:(int32_t)tag {
+  NSAssert(GPBWireFormatIsValidTag(tag), @"Invalid tag");
   switch (GPBWireFormatGetTagWireType(tag)) {
     case GPBWireFormatVarint:
       GPBCodedInputStreamReadInt32(&state_);
@@ -374,8 +381,6 @@
       SkipRawData(&state_, sizeof(int32_t));
       return YES;
   }
-  RaiseException(GPBCodedInputStreamErrorInvalidTag, nil);
-  return NO;
 }
 
 - (void)skipMessage {
diff --git a/objectivec/GPBMessage.m b/objectivec/GPBMessage.m
index 7ab184c..b9566bd 100644
--- a/objectivec/GPBMessage.m
+++ b/objectivec/GPBMessage.m
@@ -2279,6 +2279,9 @@
   while (YES) {
     BOOL merged = NO;
     tag = GPBCodedInputStreamReadTag(state);
+    if (tag == 0) {
+      break;  // Reached end.
+    }
     for (NSUInteger i = 0; i < numFields; ++i) {
       if (startingIndex >= numFields) startingIndex = 0;
       GPBFieldDescriptor *fieldDescriptor = fields[startingIndex];
@@ -2317,7 +2320,7 @@
       }
     }  // for(i < numFields)
 
-    if (!merged) {
+    if (!merged && (tag != 0)) {
       // Primitive, repeated types can be packed on unpacked on the wire, and
       // are parsed either way.  The above loop covered tag in the preferred
       // for, so this need to check the alternate form.
diff --git a/objectivec/GPBUnknownFieldSet.m b/objectivec/GPBUnknownFieldSet.m
index af08556..9ba1d65 100644
--- a/objectivec/GPBUnknownFieldSet.m
+++ b/objectivec/GPBUnknownFieldSet.m
@@ -359,6 +359,7 @@
 }
 
 - (BOOL)mergeFieldFrom:(int32_t)tag input:(GPBCodedInputStream *)input {
+  NSAssert(GPBWireFormatIsValidTag(tag), @"Got passed an invalid tag");
   int32_t number = GPBWireFormatGetTagFieldNumber(tag);
   GPBCodedInputStreamState *state = &input->state_;
   switch (GPBWireFormatGetTagWireType(tag)) {
diff --git a/objectivec/GPBWireFormat.h b/objectivec/GPBWireFormat.h
index 29cf2f0..c5941a3 100644
--- a/objectivec/GPBWireFormat.h
+++ b/objectivec/GPBWireFormat.h
@@ -53,6 +53,7 @@
     __attribute__((const));
 GPBWireFormat GPBWireFormatGetTagWireType(uint32_t tag) __attribute__((const));
 uint32_t GPBWireFormatGetTagFieldNumber(uint32_t tag) __attribute__((const));
+BOOL GPBWireFormatIsValidTag(uint32_t tag) __attribute__((const));
 
 GPBWireFormat GPBWireFormatForType(GPBDataType dataType, BOOL isPacked)
     __attribute__((const));
diff --git a/objectivec/GPBWireFormat.m b/objectivec/GPBWireFormat.m
index 193235d..860a339 100644
--- a/objectivec/GPBWireFormat.m
+++ b/objectivec/GPBWireFormat.m
@@ -49,6 +49,13 @@
   return GPBLogicalRightShift32(tag, GPBWireFormatTagTypeBits);
 }
 
+BOOL GPBWireFormatIsValidTag(uint32_t tag) {
+  uint32_t formatBits = (tag & GPBWireFormatTagTypeMask);
+  // The valid GPBWireFormat* values are 0-5, anything else is not a valid tag.
+  BOOL result = (formatBits <= 5);
+  return result;
+}
+
 GPBWireFormat GPBWireFormatForType(GPBDataType type, BOOL isPacked) {
   if (isPacked) {
     return GPBWireFormatLengthDelimited;