Undefined reference to rapidjson::Value - c++

Question

I have a project which uses rapidjson, which compiles well on OS X with both cmake and Xcode.
I'm puzzled why I get the following undefined reference error (output in German, "Nicht definierter Verweis auf" means "undefined reference to":
../libraries/hdm-concave-lib/build/libhdm-concave-lib.a(geojson2pointcloud.cpp.o): In Funktion `hdm::concave::Geojson2PointCloud::toPointCloud(std::string, double)':
geojson2pointcloud.cpp:(.text+0xb4a): Nicht definierter Verweis auf `GeojsonParser::parseGeometry(rapidjson::GenericValue<rapidjson::UTF8<char>, rapidjson::MemoryPoolAllocator<rapidjson::CrtAllocator> >&)'
collect2: error: ld returned 1 exit status
hdm-concave/CMakeFiles/hdm-concave.dir/build.make:96: die Regel für Ziel „hdm-concave/hdm-concave“ scheiterte
make[2]: *** [hdm-concave/hdm-concave] Fehler 1
CMakeFiles/Makefile2:85: die Regel für Ziel „hdm-concave/CMakeFiles/hdm-concave.dir/all“ scheiterte
make[1]: *** [hdm-concave/CMakeFiles/hdm-concave.dir/all] Fehler 2
Makefile:83: die Regel für Ziel „all“ scheiterte
make: *** [all] Fehler 2
The header to the method in question looks like this:
class GeojsonParser
{
public:
...
bool parseGeometry(rapidjson::Value& geometry);
}
And the implementation is this:
bool GeojsonParser::parseGeometry(rapidjson::Value &geometry) {
_isGeometryCollection = false;
bool res = parseFeatureType(geometry);
return res;
}
If I comment out the line with _parser.parseGeometry the project compiles and links successfully (though of course the program isn't doing anything useful).
for (Value::ValueIterator itr = features.Begin(); itr != features.End(); ++itr) {
if (!itr->HasMember("geometry")) continue;
if (itr->HasMember("geometry")) {
Value& geomVal = (*itr)["geometry"];
_parser.parseGeometry(geomVal); // <-- linker error here
}
}
I do not see anything I'm doing wrong, but I suspect it's something to do with rapidjson which I do not get. Did you ever encounter the same issue and know how to fix it?