diff --git a/superset/db_engine_specs/base.py b/superset/db_engine_specs/base.py
index 3767cdf541597..37e10e27419b8 100644
--- a/superset/db_engine_specs/base.py
+++ b/superset/db_engine_specs/base.py
@@ -743,7 +743,10 @@ def fetch_data(
                 return cursor.fetchmany(limit)
             data = cursor.fetchall()
             description = cursor.description or []
-            column_type_mutators = {
+            # Create a mapping between column name and a mutator function to normalize
+            # values with. The first two items in the description row are
+            # the column name and type.
+            column_mutators = {
                 row[0]: func
                 for row in description
                 if (
@@ -752,11 +755,11 @@ def fetch_data(
                     )
                 )
             }
-            if column_type_mutators:
+            if column_mutators:
                 indexes = {row[0]: idx for idx, row in enumerate(description)}
                 for row_idx, row in enumerate(data):
                     new_row = list(row)
-                    for col, func in column_type_mutators.items():
+                    for col, func in column_mutators.items():
                         col_idx = indexes[col]
                         new_row[col_idx] = func(row[col_idx])
                     data[row_idx] = tuple(new_row)
diff --git a/tests/unit_tests/db_engine_specs/test_mysql.py b/tests/unit_tests/db_engine_specs/test_mysql.py
index dad00034af05d..7e593e04c8f42 100644
--- a/tests/unit_tests/db_engine_specs/test_mysql.py
+++ b/tests/unit_tests/db_engine_specs/test_mysql.py
@@ -236,6 +236,11 @@ def test_get_schema_from_engine_params() -> None:
             [("dec", "decimal(12,6)"), ("str", "varchar(3)")],
             [(Decimal("1.23456"), "abc")],
         ),
+        (
+            [[None, "abc"]],
+            [("dec", "decimal(12,6)"), ("str", "varchar(3)")],
+            [(None, "abc")],
+        ),
         (
             [["1.23456", "abc"]],
             [("dec", "varchar(255)"), ("str", "varchar(3)")],
diff --git a/tests/unit_tests/db_engine_specs/test_trino.py b/tests/unit_tests/db_engine_specs/test_trino.py
index 7ddbd4f1f9f4a..d2e75a0842292 100644
--- a/tests/unit_tests/db_engine_specs/test_trino.py
+++ b/tests/unit_tests/db_engine_specs/test_trino.py
@@ -384,6 +384,11 @@ def test_handle_cursor_early_cancel(
             [("dec", "decimal(12,6)"), ("str", "varchar(3)")],
             [(Decimal("1.23456"), "abc")],
         ),
+        (
+            [[None, "abc"]],
+            [("dec", "decimal(12,6)"), ("str", "varchar(3)")],
+            [(None, "abc")],
+        ),
         (
             [["1.23456", "abc"]],
             [("dec", "varchar(255)"), ("str", "varchar(3)")],