diff --git a/hera_opm/mf_tools.py b/hera_opm/mf_tools.py
index d33aff9..5cb1775 100644
--- a/hera_opm/mf_tools.py
+++ b/hera_opm/mf_tools.py
@@ -1440,7 +1440,7 @@ def make_lstbin_config_file(config, outdir: str) -> int:
 
     from hera_cal.lst_stack.config import LSTBinConfiguration
 
-    lstconfig = LSTBinConfiguration.from_toml(toml.dump(lstconfig))
+    lstconfig = LSTBinConfiguration.from_toml(toml.dumps(lstconfig))
     print(f"Found {len(lstconfig.data_files)} nights of data.")
     print("Each night has the following number of files:")
     for flist in lstconfig.data_files: