Beamer: Problem With Capital Greek Letters [closed]

I am using beamer and the capital greek letters are displayed as black boxes.I have found this error in other questions but none of the answers worked for me.Here is the code

\documentclass[]{beamer}\usepackage[utf8]{inputenc}\usepackage[spanish]{babel} \usepackage{hyperref}\usepackage{amsmath}\usepackage{amssymb}% \usepackage{amsthm}% \usepackage{mathabx} % conflict with amssymb \usepackage{mathrsfs}\begin{document}\begin{frame}[fragile] \titles{Propositional Logic}{}  \begin{small}   foo $\gamma$ $\Gamma$  \end{small} \end{frame}\end{document}