diff --git a/build/fbcode_builder/getdeps/dyndeps.py b/build/fbcode_builder/getdeps/dyndeps.py
index 47ff13766..abc02e7e9 100644
--- a/build/fbcode_builder/getdeps/dyndeps.py
+++ b/build/fbcode_builder/getdeps/dyndeps.py
@@ -251,7 +251,10 @@ class MachDeps(DepBase):
         with open(objfile, "rb") as f:
             # mach stores the magic number in native endianness,
             # so unpack as native here and compare
-            magic = unpack("I", f.read(4))[0]
+            header = f.read(4)
+            if len(header) != 4:
+                return False
+            magic = unpack("I", header)[0]
             return magic == MACH_MAGIC
 
     def list_dynamic_deps(self, objfile):