longmapsto strange space

The problem lies on the package 'breqn'. Once removed, the symbol appears correct.

documentclass[runningheads]{llncs}

usepackage{amsmath}
usepackage{amssymb}
usepackage{breqn}  

begin{document}    

begin{align}       
   F: mathcal{D} &longrightarrow mathbb{R}^d nonumber  
   x & longmapsto textbf{textit{m}}_i    
end{align} 
 
end{document}