diff --git a/projects/pyyaml/Dockerfile b/projects/pyyaml/Dockerfile
index d555b402a..21ca7f718 100644
--- a/projects/pyyaml/Dockerfile
+++ b/projects/pyyaml/Dockerfile
@@ -16,6 +16,7 @@
 
 FROM gcr.io/oss-fuzz-base/base-builder-python
 RUN git clone https://github.com/yaml/pyyaml
+RUN python3 -m pip install --upgrade pip
 WORKDIR $SRC
 COPY build.sh $SRC/
 COPY fuzz_* $SRC/
diff --git a/projects/pyyaml/build.sh b/projects/pyyaml/build.sh
index b5fbb755f..5ca219bfc 100644
--- a/projects/pyyaml/build.sh
+++ b/projects/pyyaml/build.sh
@@ -15,7 +15,8 @@
 #
 ################################################################################
 cd pyyaml
-pip3 install .
+echo 'Cython < 3.0' > /tmp/constraint.txt
+PIP_CONSTRAINT=/tmp/constraint.txt pip3 install .
 
 # Build fuzzers in $OUT.
 for fuzzer in $(find $SRC -name 'fuzz_*.py'); do