diff --git a/src/pineko/fonll.py b/src/pineko/fonll.py
index b3b250e5..e9fe54ea 100644
--- a/src/pineko/fonll.py
+++ b/src/pineko/fonll.py
@@ -137,7 +137,6 @@ def theorycard_no_fns_pto(self):
                     continue
                 if card[key] != theorycards[0][key]:
                     differ.append(key)
-                del theorycards[0][key]
 
             if len(differ) > 0:
                 raise ValueError(
diff --git a/src/pineko/theory.py b/src/pineko/theory.py
index 8286fabe..5cb53f78 100644
--- a/src/pineko/theory.py
+++ b/src/pineko/theory.py
@@ -23,6 +23,30 @@
 logger = logging.getLogger(__name__)
 
 
+def _check_for_scale_variations(tcard, grid_path):
+    """Check that the grid is compatible with the requested scale_variations (if any).
+
+    Parameters
+    ----------
+    tcard : dict
+        theory card
+    grid_path : pathlib.Path
+        path to grid
+    """
+    grid = pineappl.grid.Grid.read(grid_path)
+    xir = tcard["XIR"]
+    xif = tcard["XIF"]
+    max_al = 0  # We don't do SV for alpha
+    max_as = 1 + int(tcard["PTO"])
+    # In FONLL-B we might need to change max_as
+    if check.is_fonll_mixed(tcard["FNS"], grid.convolutions):
+        max_as += 1
+    if not np.isclose(xir, 1.0):
+        check_scvar_evolve(grid, max_as, max_al, check.Scale.REN)
+    if not np.isclose(xif, 1.0) and evolve.sv_scheme(tcard) is None:
+        check_scvar_evolve(grid, max_as, max_al, check.Scale.FACT)
+
+
 def get_eko_names(grid_path, name, filter=True):
     """Get the names of the ekos depending on the types of convolutions.
 
@@ -276,6 +300,9 @@ def opcard(self, name, grid, tcard):
         tcard : dict
             theory card
         """
+        # Before creating the operator card, check whether this grid is legal or not
+        _check_for_scale_variations(tcard, grid)
+
         opcard_path = self.operator_cards_path / f"{name}.yaml"
         if opcard_path.exists():
             if not self.overwrite:
@@ -436,7 +463,6 @@ def fk(self, name, grid_path, tcard, pdfs):
 
         # check if grid contains SV if theory is requesting them (in particular
         # if theory is requesting scheme A or C)
-        sv_method = evolve.sv_scheme(tcard)
         xir = tcard["XIR"]
         xif = tcard["XIF"]
         xia = 1.0  # TODO: modify into `tcard["XIA"]`
@@ -473,12 +499,6 @@ def fk(self, name, grid_path, tcard, pdfs):
                 rich.print("[green] Skipping empty grid.")
                 return
 
-        # check for sv
-        if not np.isclose(xir, 1.0):
-            check_scvar_evolve(grid, max_as, max_al, check.Scale.REN)
-        if sv_method is None:
-            if not np.isclose(xif, 1.0):
-                check_scvar_evolve(grid, max_as, max_al, check.Scale.FACT)
         # TODO: Add fragmentation scale variations
         # loading ekos to produce a tmp copy
         n_ekos = len(eko_filename)