out of bound memory access of yylsp

[yuansong]

HI， After generating a rule with flex/bison, when using a code checking tool 
reported the following error:
OUT of bound memory access (accessed memory precedes memory block in yylsp[-4])


C Code:
(yyval.typname)->location = (yylsp[-4])


gram.y:
get a location of express, just like :
typename->location =@4;


I suspect that it may be necessary to set a certain option. Does anyone know 
about this? help me! Thank you!