String concatenation segmentation error

[tcak]

string fileContent = "";

...

writeln(ri, ": debug 1");
foreach(i; 0..dim)
{
	if( i > 0 ){ fileContent ~= "\t"; }

	writeln(ri, ": debug 1.1: ", ri*dim + i, ": ", positions[ ri*dim 
+ i ]);

	fileContent ~= to!string(positions[ ri*dim + i ]);

	writeln(ri, ": debug 1.2: ", ri*dim + i, ": ", positions[ ri*dim 
+ i ]);
}

---------

On line "fileContent ~= ...", I get a segmentation fault.

"positions" array is defined as auto positions = new float[ 100 
]; So, I am 100% sure, it is not out of range. "ri*dim + 1" is 
not a big number at all.

...
4: debug 1.1: 9: 0.271075
4: debug 1.2: 9: 0.271075
4: debug 2
4: debug 2.1: 4
4: debug 3
4: debug 4
5: debug 1
5: debug 1.1: 10: 0.884978
5: debug 1.2: 10: 0.884978
5: debug 1.1: 11: 0.813104
Segmentation fault
...

I have compiled the code with "-g" flag and ran it with GNU 
debugger. It gives following:

Thread 1 "dataspace" received signal SIGSEGV, Segmentation fault.
0x00005555556ca286 in 
_D2gc4impl12conservativeQw3Gcx10smallAllocMFNbmKmkxC8TypeInfoZPv 
()


So, there is a problem about small allocation. I remember I had 
this problem before in another project.

I have enough free ram. htop shows 3.96 GiB of 8 GiB is used only 
and swap is not in use.

DMD64 D Compiler v2.094.0

Is this error related to me? Is it a programmer error? Is it a 
bug? Am I doing something wrong? This is a compiler related 
operation (string concatenation), and I assume/expect that it 
would work without a problem.