Crash report says game is modded but it isn't

Hi, hope this isn't already reported, I checked but can't be 100% sure.

I started playing Minecraft in a superflat map, made of only one layer of sand. No surprises on what's next, the sand collapsed after I but a block on it, and ten or twenty seconds after, the game crash.

What I expected to happen was...:
I expect the crash report pointed a lack of power in my computer.

What actually happened was...:
The crash report says it isn't possible to report because my game may be modded, which it isn't at all.

Steps to Reproduce:
1. Have a computer which is not very powerful with its CPU.
2. Open a superflat map made of one layer of sand
3. Make the sand collapse.

NB : I report the fact that the launcher is wrong about a possibly modded version, not about the crash itself.

Sorry for the english, bad me.