Pygame rendering colour blocks in tiles with transparency


My friends and I are trying to develop a tile based game for the Raspberry Pi using Pygame using Tiled for map creation. Pygame has no problem rendering tiles with no transparency. However it adds colour blocks to the transparent part of tiles in the upper layers. What could be causing this? There are no custom properties added to the grass tiles

This is what the tiles look like in Tiled

This is they look like in Pygame