diff --git a/fastboot/usb_osx.cpp b/fastboot/usb_osx.cpp
index 610eebf..a4b9307 100644
--- a/fastboot/usb_osx.cpp
+++ b/fastboot/usb_osx.cpp
@@ -191,16 +191,30 @@
 
         // Iterate over the endpoints for this interface and see if there
         // are any that do bulk in/out.
-        for (UInt8 endpoint = 1; endpoint <= interfaceNumEndpoints; endpoint++) {
+        for (UInt8 endpoint = 1; endpoint <= interfaceNumEndpoints; ++endpoint) {
             UInt8   transferType;
-            UInt16  maxPacketSize;
+            UInt16  endPointMaxPacketSize = 0;
             UInt8   interval;
+
+            // Attempt to retrieve the 'true' packet-size from supported interface.
+            kr = (*interface)
+                 ->GetEndpointProperties(interface, 0, endpoint,
+                                       kUSBOut,
+                                       &transferType,
+                                       &endPointMaxPacketSize, &interval);
+            if (kr == kIOReturnSuccess && !endPointMaxPacketSize) {
+                ERR("GetEndpointProperties() returned zero len packet-size");
+            }
+
+            UInt16  pipePropMaxPacketSize;
             UInt8   number;
             UInt8   direction;
 
+            // Proceed with extracting the transfer direction, so we can fill in the
+            // appropriate fields (bulkIn or bulkOut).
             kr = (*interface)->GetPipeProperties(interface, endpoint,
                     &direction,
-                    &number, &transferType, &maxPacketSize, &interval);
+                    &number, &transferType, &pipePropMaxPacketSize, &interval);
 
             if (kr == 0) {
                 if (transferType != kUSBBulk) {
@@ -216,7 +230,8 @@
                 }
 
                 if (handle->info.ifc_protocol == 0x01) {
-                    handle->zero_mask = maxPacketSize - 1;
+                    handle->zero_mask = (endPointMaxPacketSize == 0) ?
+                        pipePropMaxPacketSize - 1 : endPointMaxPacketSize - 1;
                 }
             } else {
                 ERR("could not get pipe properties for endpoint %u (%08x)\n", endpoint, kr);