diff --git a/spextra/libraries.py b/spextra/libraries.py
index 641f947..939c3bb 100644
--- a/spextra/libraries.py
+++ b/spextra/libraries.py
@@ -45,30 +45,36 @@ def __init__(self, name):
         self._items = yamldict.get(self.aliases["items"], {})
         self.spectral_coverage = yamldict.get("spectral_coverage", [])
 
-        self.read_kwargs = self._get_read_kwargs(yamldict)
-        self._validate_units()
-
         self.file_extension = yamldict.get("file_extension", None)
+        self.read_kwargs = self._get_read_kwargs(yamldict)
 
     def _get_read_kwargs(self, yamldict) -> dict:
         read_kwargs = {
             "wave_col": yamldict.get(self.aliases["wave_col"], "WAVELENGTH"),
             "flux_col": yamldict.get(self.aliases["flux_col"], "FLUX"),
-            "wave_unit": yamldict.get("wave_unit", "Angstrom"),
-            "flux_unit": yamldict.get(self.aliases["flux_unit"], "FLAM"),
         }
+
+        # FITS stores units in TUNIT keywords
+        if self.data_type != "fits":
+            read_kwargs.update({
+                "wave_unit": yamldict.get("wave_unit", "Angstrom"),
+                "flux_unit": yamldict.get(self.aliases["flux_unit"], "FLAM"),
+            })
+            read_kwargs = self._validate_units(read_kwargs)
         return read_kwargs
 
-    def _validate_units(self) -> None:
+    @staticmethod
+    def _validate_units(read_kwargs: dict) -> dict:
         for key in ("wave_unit", "flux_unit"):
-            value = self.read_kwargs[key]
+            value = read_kwargs[key]
             # HACK to understand ext. units
             if value == "Av/E(B-V)":
                 value = "mag"
             try:
-                self.read_kwargs[key] = validate_unit(value)
+                read_kwargs[key] = validate_unit(value)
             except (SynphotError, ValueError) as err:
                 raise UnitError(f"{value} not understood.") from err
+        return read_kwargs
 
     @property
     def is_in_database(self) -> bool: