"C and C++ compiler paths differ. C compiler may not work."

When starting Qt Creator, I get warnings in General Messages:

C and C++ compiler paths differ. C compiler may not work.
C and C++ compiler paths differ. C compiler may not work.

These warnings come from QbsProjectManager. When augmenting the code with qDebug, it looks like the path of the C compiler is empty.

Anyhow, if I check out the Kit, everything is configured correctly.