diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index 6fccd029..950a5f0c 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -32,6 +32,9 @@ jobs:
           override: true
       - run: cargo test
       - run: cargo test --features integer128
+      # Downgrade the indexmap dependency for MSRV 1.64
+      - run: cargo update -p indexmap --precise 2.5.0
+        if: ${{ matrix.rust == '1.64.0' }}
       - run: cargo test --features indexmap
       - run: cargo test --all-features
 
@@ -52,6 +55,8 @@ jobs:
           override: true
       - run: cargo clippy -- -D warnings
       - run: cargo clippy --features integer128 -- -D warnings
+      # Downgrade the indexmap dependency for MSRV 1.64
+      - run: cargo update -p indexmap --precise 2.5.0
       - run: cargo clippy --features indexmap -- -D warnings
       - run: cargo clippy --all-features -- -D warnings