diff --git a/geopandas/io/sql.py b/geopandas/io/sql.py
index 9bd0f1d..17e7c40 100644
--- a/geopandas/io/sql.py
+++ b/geopandas/io/sql.py
@@ -230,7 +230,7 @@ def _get_geometry_type(gdf):
 
     # Check for 3D-coordinates
     if any(gdf.geometry.has_z):
-        target_geom_type = target_geom_type + "Z"
+        target_geom_type += "Z"
 
     return target_geom_type, has_curve
 
diff --git a/geopandas/tools/overlay.py b/geopandas/tools/overlay.py
index babb67a..15133ef 100644
--- a/geopandas/tools/overlay.py
+++ b/geopandas/tools/overlay.py
@@ -139,7 +139,7 @@ def _overlay_union(df1, df2):
     # keep geometry column last
     columns = list(dfunion.columns)
     columns.remove("geometry")
-    columns = columns + ["geometry"]
+    columns.append("geometry")
     return dfunion.reindex(columns=columns)