diff --git a/infra/base-images/base-builder/Dockerfile b/infra/base-images/base-builder/Dockerfile
index 313bf8507..356e94e9a 100644
--- a/infra/base-images/base-builder/Dockerfile
+++ b/infra/base-images/base-builder/Dockerfile
@@ -22,7 +22,7 @@ RUN apt-get install -y git subversion jq python3 zip make libunwind8-dev binutil
 ENV SANITIZER_FLAGS_address "-fsanitize=address -fsanitize-address-use-after-scope"
 ENV SANITIZER_FLAGS_undefined "-fsanitize=bool,array-bounds,float-divide-by-zero,function,integer-divide-by-zero,return,shift,signed-integer-overflow,vla-bound,vptr -fno-sanitize-recover=undefined"
 ENV SANITIZER_FLAGS_memory "-fsanitize=memory -fsanitize-memory-track-origins"
-ENV SANITIZER_FLAGS_profile "-fprofile-instr-generate -fcoverage-mapping"
+ENV SANITIZER_FLAGS_profile "-fprofile-instr-generate -fcoverage-mapping -pthread"
 
 # Default build flags for coverage.
 ENV COVERAGE_FLAGS="-fsanitize-coverage=trace-pc-guard,trace-cmp"
diff --git a/infra/base-images/base-builder/compile b/infra/base-images/base-builder/compile
index 089813152..434f526f1 100755
--- a/infra/base-images/base-builder/compile
+++ b/infra/base-images/base-builder/compile
@@ -33,8 +33,8 @@ then
 fi
 
 # Coverage flag overrides.
-COVERAGE_FLAGS_VAR="COVERAGE_FLAGS_$SANITIZER"
-if [[ -n ${!COVERAGE_FLAGS_VAR-} ]]
+COVERAGE_FLAGS_VAR="COVERAGE_FLAGS_${SANITIZER}"
+if [[ -n ${!COVERAGE_FLAGS_VAR+x} ]]
 then
   export COVERAGE_FLAGS="${!COVERAGE_FLAGS_VAR}"
 fi