diff --git a/bin/ruby-build b/bin/ruby-build
index 59b43a40..bcb48b09 100755
--- a/bin/ruby-build
+++ b/bin/ruby-build
@@ -246,24 +246,39 @@ compute_md5() {
   fi
 }
 
+has_checksum_support() {
+  local checksum_command="$1"
+  local has_checksum_var="HAS_CHECKSUM_SUPPORT_${checksum_command}"
+
+  if [ -z "${!has_checksum_var+defined}" ]; then
+    printf -v "$has_checksum_var" "$(echo test | "$checksum_command" >/dev/null; echo $?)"
+  fi
+  return "${!has_checksum_var}"
+}
+
 verify_checksum() {
-  # If there's no SHA2 support, return success
-  [ -n "$HAS_SHA2_SUPPORT" ] || return 0
-  local checksum_command="compute_sha2"
+  local checksum_command
+  local filename="$1"
+  local expected_checksum="$(echo "$2" | tr [A-Z] [a-z])"
 
   # If the specified filename doesn't exist, return success
-  local filename="$1"
   [ -e "$filename" ] || return 0
 
-  # If there's no expected checksum, return success
-  local expected_checksum=`echo "$2" | tr [A-Z] [a-z]`
-  [ -n "$expected_checksum" ] || return 0
+  case "${#expected_checksum}" in
+  0) return 0 ;; # empty checksum; return success
+  32) checksum_command="compute_md5" ;;
+  64) checksum_command="compute_sha2" ;;
+  *)
+    { echo
+      echo "unexpected checksum length: ${#expected_checksum} (${expected_checksum})"
+      echo "expected 0 (no checksum), 32 (MD5), or 64 (SHA2-256)"
+      echo
+    } >&4
+    return 1 ;;
+  esac
 
-  # If the checksum length is 32 chars, assume MD5, otherwise SHA2
-  if [ "${#expected_checksum}" -eq 32 ]; then
-    [ -n "$HAS_MD5_SUPPORT" ] || return 0
-    checksum_command="compute_md5"
-  fi
+  # If chosen provided checksum algorithm isn't supported, return success
+  has_checksum_support "$checksum_command" || return 0
 
   # If the computed checksum is empty, return failure
   local computed_checksum=`echo "$($checksum_command < "$filename")" | tr [A-Z] [a-z]`
@@ -1219,23 +1234,10 @@ else
   RUBY_BUILD_DEFAULT_MIRROR=
 fi
 
-if [ -n "$RUBY_BUILD_SKIP_MIRROR" ]; then
+if [ -n "$RUBY_BUILD_SKIP_MIRROR" ] || ! has_checksum_support compute_sha2; then
   unset RUBY_BUILD_MIRROR_URL
 fi
 
-if echo test | compute_sha2 >/dev/null; then
-  HAS_SHA2_SUPPORT=1
-else
-  unset HAS_SHA2_SUPPORT
-  unset RUBY_BUILD_MIRROR_URL
-fi
-
-if echo test | compute_md5 >/dev/null; then
-  HAS_MD5_SUPPORT=1
-else
-  unset HAS_MD5_SUPPORT
-fi
-
 SEED="$(date "+%Y%m%d%H%M%S").$$"
 LOG_PATH="${TMP}/ruby-build.${SEED}.log"
 RUBY_BIN="${PREFIX_PATH}/bin/ruby"
diff --git a/test/cache.bats b/test/cache.bats
index db619552..2c5bb1bf 100644
--- a/test/cache.bats
+++ b/test/cache.bats
@@ -10,7 +10,6 @@ setup() {
 
 
 @test "packages are saved to download cache" {
-  stub shasum true
   stub curl "-q -o * -*S* http://example.com/* : cp $FIXTURE_ROOT/\${5##*/} \$3"
 
   install_fixture definitions/without-checksum
@@ -19,12 +18,10 @@ setup() {
   [ -e "${RUBY_BUILD_CACHE_PATH}/package-1.0.0.tar.gz" ]
 
   unstub curl
-  unstub shasum
 }
 
 
 @test "cached package without checksum" {
-  stub shasum true
   stub curl
 
   cp "${FIXTURE_ROOT}/package-1.0.0.tar.gz" "$RUBY_BUILD_CACHE_PATH"
@@ -35,7 +32,6 @@ setup() {
   [ -e "${RUBY_BUILD_CACHE_PATH}/package-1.0.0.tar.gz" ]
 
   unstub curl
-  unstub shasum
 }
 
 
@@ -79,7 +75,6 @@ setup() {
 
 
 @test "nonexistent cache directory is ignored" {
-  stub shasum true
   stub curl "-q -o * -*S* http://example.com/* : cp $FIXTURE_ROOT/\${5##*/} \$3"
 
   export RUBY_BUILD_CACHE_PATH="${TMP}/nonexistent"
@@ -91,5 +86,4 @@ setup() {
   [ ! -d "$RUBY_BUILD_CACHE_PATH" ]
 
   unstub curl
-  unstub shasum
 }
diff --git a/test/checksum.bats b/test/checksum.bats
index bcc630b0..a762b4f1 100644
--- a/test/checksum.bats
+++ b/test/checksum.bats
@@ -6,7 +6,6 @@ export RUBY_BUILD_CACHE_PATH=
 
 
 @test "package URL without checksum" {
-  stub shasum true
   stub curl "-q -o * -*S* http://example.com/* : cp $FIXTURE_ROOT/\${5##*/} \$3"
 
   install_fixture definitions/without-checksum
@@ -14,7 +13,6 @@ export RUBY_BUILD_CACHE_PATH=
   [ -x "${INSTALL_ROOT}/bin/package" ]
 
   unstub curl
-  unstub shasum
 }
 
 
@@ -143,3 +141,16 @@ DEF
 
   unstub shasum
 }
+
+@test "package URL with checksum of unexpected length" {
+  stub curl "-q -o * -*S* http://example.com/* : cp $FIXTURE_ROOT/\${5##*/} \$3"
+
+  run_inline_definition <<DEF
+install_package "package-1.0.0" "http://example.com/packages/package-1.0.0.tar.gz#checksum_of_unexpected_length" copy
+DEF
+
+  assert_failure
+  [ ! -f "${INSTALL_ROOT}/bin/package" ]
+  assert_output_contains "unexpected checksum length: 29 (checksum_of_unexpected_length)"
+  assert_output_contains "expected 0 (no checksum), 32 (MD5), or 64 (SHA2-256)"
+}
diff --git a/test/fixtures/definitions/with-invalid-checksum b/test/fixtures/definitions/with-invalid-checksum
index 1183810f..bb2572a9 100644
--- a/test/fixtures/definitions/with-invalid-checksum
+++ b/test/fixtures/definitions/with-invalid-checksum
@@ -1 +1 @@
-install_package "package-1.0.0" "http://example.com/packages/package-1.0.0.tar.gz#invalid" copy
+install_package "package-1.0.0" "http://example.com/packages/package-1.0.0.tar.gz#invalid_64_character_checksum_0000000000000000000000000000000000" copy