diff --git a/src/ert/config/design_matrix.py b/src/ert/config/design_matrix.py
index ee8b611406d..f8a3b44e671 100644
--- a/src/ert/config/design_matrix.py
+++ b/src/ert/config/design_matrix.py
@@ -221,14 +221,23 @@ def _read_defaultssheet(
             return {}
         if len(default_df.columns) < 2:
             raise ValueError("Defaults sheet must have at least two columns")
+        empty_cells = [
+            f"Row {default_df.index[i]}, column {default_df.columns[j]}"
+            for i, j in zip(*np.where(pd.isna(default_df)))
+        ]
+        if empty_cells > 0:
+            raise ValueError(f"Default sheet contains empty cells {empty_cells}")
         # Look for initial or trailing whitespace in parameter names. This
         # is disallowed as it can create user confusion and has no use-case.
+        whitespace_errors = []
         for paramname in default_df.loc[:, 0]:
             if paramname != paramname.strip():
-                raise ValueError(
+                whitespace_errors.append(
                     f"Parameter name '{paramname}' in default values contains "
                     "initial or trailing whitespace."
                 )
+        if whitespace_errors > 0:
+            raise ValueError(whitespace_errors)
 
         return {row[0]: convert_to_numeric(row[1]) for _, row in default_df.iterrows()}