diff --git a/yt/frontends/flash/data_structures.py b/yt/frontends/flash/data_structures.py
index 4bc55386a42..7d617f07a1e 100644
--- a/yt/frontends/flash/data_structures.py
+++ b/yt/frontends/flash/data_structures.py
@@ -383,6 +383,10 @@ def _parse_parameter_file(self):
             nblockx = self.parameters["nblockx"]
             nblocky = self.parameters["nblocky"]
             nblockz = self.parameters["nblockz"]
+        elif self.parameters["globalnumblocks"] == 1:  # non-fixed block size UG
+            nblockx = 1
+            nblocky = 1
+            nblockz = 1
         else:  # Uniform Grid
             nblockx = self.parameters["iprocs"]
             nblocky = self.parameters["jprocs"]