diff --git a/pyam/core.py b/pyam/core.py
index 168bf4c28..81539a16c 100755
--- a/pyam/core.py
+++ b/pyam/core.py
@@ -989,6 +989,7 @@ def categorize(
 
         # update meta dataframe
         self._new_meta_column(name)
+        self.meta[name] = self.meta[name].astype(object)
         self.meta.loc[idx, name] = value
         msg = "{} scenario{} categorized as `{}: {}`"
         logger.info(msg.format(len(idx), "" if len(idx) == 1 else "s", name, value))
diff --git a/pyam/statistics.py b/pyam/statistics.py
index 44e94149c..673dc3709 100644
--- a/pyam/statistics.py
+++ b/pyam/statistics.py
@@ -290,6 +290,7 @@ def format_rows(
     count = max(
         [i for i in row.loc[(slice(None), slice(None), "count")] if not np.isnan(i)]
     )
+    ret = ret.astype(object)
     ret.loc[("count", "")] = (f"{count:.0f}") if count > 1 else ""
 
     # set upper and lower for the range
diff --git a/tests/test_core.py b/tests/test_core.py
index 73a56e05a..371b6994f 100644
--- a/tests/test_core.py
+++ b/tests/test_core.py
@@ -92,6 +92,7 @@ def test_init_df_with_duplicates_raises(test_df):
 def test_init_df_with_illegal_values_raises(test_pd_df, illegal_value):
     # values that cannot be cast to float should raise a value error and be specified by
     # index for user
+    test_pd_df[2005] = test_pd_df[2005].astype(object)
     test_pd_df.loc[0, 2005] = illegal_value
     msg = (
         f'.*string "{illegal_value}" in `data`:'