Invalid geometry - lines

In QGIS I’m attempting to create a buffer zone around a number of connected lines, then use the buffer layer to clip another layer. The lines features were provided as a shape file, and buffering works fine. However when I try to clip with the buffered layer I get an error stating ‘invalid geometry’. Upon closer inspection, there is a line not connected to the others (see image), which appears to be the source of the error. How do I connect this line to the other? I’ve experimented with edit features (extend, reshape etc.) but can’t seem to find the function I require to link them.