diff --git a/.github/workflows/testing.yml b/.github/workflows/testing.yml
index c964234..c83e398 100644
--- a/.github/workflows/testing.yml
+++ b/.github/workflows/testing.yml
@@ -7,6 +7,9 @@ on:
 jobs:
   testing:
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        react-version: [18, 19]
     env:
       BS_ACCESS_KEY: ${{ secrets.BS_ACCESS_KEY }}
       BS_USERNAME: ${{ secrets.BS_USERNAME }}
@@ -20,6 +23,8 @@ jobs:
           node-version: 14
       - name: Installing Dependencies
         run: yarn install --frozen-lockfile
+      - name: Install React ${{ matrix.react-version }}
+        run: yarn add react@${{ matrix.react-version }} react-dom@${{ matrix.react-version }} --dev
       - name: Checking File Size
         run: yarn check:size
       - name: Checking Linting Rules
diff --git a/package.json b/package.json
index f30f1a3..56b0aa6 100644
--- a/package.json
+++ b/package.json
@@ -72,8 +72,8 @@
     "package.json"
   ],
   "peerDependencies": {
-    "react": "16.8.0 - 18",
-    "react-dom": "16.8.0 - 18"
+    "react": "^16.8.0 || ^17.0.0 || ^18.0.0 || ^19.0.0",
+    "react-dom": "^16.8.0 || ^17.0.0 || ^18.0.0 || ^19.0.0"
   },
   "devDependencies": {
     "@babel/core": "^7.7.7",
@@ -95,8 +95,8 @@
     "@types/jest": "^28.1.1",
     "@types/karma": "^6.3.1",
     "@types/karma-jasmine": "^4.0.1",
-    "@types/react": "^18.0.8",
-    "@types/react-dom": "^18.0.3",
+    "@types/react": "^19.0.0",
+    "@types/react-dom": "^19.0.0",
     "@typescript-eslint/eslint-plugin": "^5.27.1",
     "@typescript-eslint/parser": "^5.27.1",
     "babel-loader": "^8.1.0",
@@ -117,9 +117,9 @@
     "lint-staged": "^13.0.1",
     "npm-run-all": "^4.1.5",
     "prettier": "^2.0.4",
-    "react": "^18.0.0",
+    "react": "^19.0.0",
     "react-app-polyfill": "^3.0.0",
-    "react-dom": "^18.0.0",
+    "react-dom": "^19.0.0",
     "rollup": "^2.6.1",
     "semantic-release": "^19.0.3",
     "size-limit": "^5.0.1",
diff --git a/src/index.ts b/src/index.ts
index 21d1350..d3c8ea7 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -38,7 +38,7 @@ export type RoundingFunction = (n: number) => number;
 
 function useResizeObserver<T extends Element>(
   opts: {
-    ref?: RefObject<T> | T | null | undefined;
+    ref?: RefObject<T | null | undefined> | T | null | undefined;
     onResize?: ResizeHandler;
     box?: ResizeObserverBoxOptions;
     round?: RoundingFunction;
@@ -57,7 +57,7 @@ function useResizeObserver<T extends Element>(
     box?: ResizeObserverBoxOptions;
     round?: RoundingFunction;
     instance: ResizeObserver;
-  }>();
+  }>(undefined);
 
   const [size, setSize] = useState<{
     width?: number;
diff --git a/src/utils/useResolvedElement.ts b/src/utils/useResolvedElement.ts
index d2acd46..bcc980b 100644
--- a/src/utils/useResolvedElement.ts
+++ b/src/utils/useResolvedElement.ts
@@ -7,10 +7,10 @@ type SubscriberResponse = SubscriberCleanupFunction | void;
 // refs to such extent, but then composing hooks and components could not opt out of unnecessary renders.
 export default function useResolvedElement<T extends Element>(
   subscriber: (element: T) => SubscriberResponse,
-  refOrElement?: T | RefObject<T> | null
+  refOrElement?: T | RefObject<T | null | undefined> | null
 ): RefCallback<T> {
   const lastReportRef = useRef<{
-    element: T | null;
+    element: T | null | undefined;
     subscriber: typeof subscriber;
     cleanup?: SubscriberResponse;
   } | null>(null);
@@ -28,7 +28,7 @@ export default function useResolvedElement<T extends Element>(
     const cbElement = cbElementRef.current;
     const refOrElement = refOrElementRef.current;
     // Ugly ternary. But smaller than an if-else block.
-    const element: T | null = cbElement
+    const element: T | null | undefined = cbElement
       ? cbElement
       : refOrElement
       ? refOrElement instanceof Element
diff --git a/yarn.lock b/yarn.lock
index 6a30a29..3578c81 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -1996,14 +1996,19 @@
   resolved "https://registry.yarnpkg.com/@types/prop-types/-/prop-types-15.7.5.tgz#5f19d2b85a98e9558036f6a3cacc8819420f05cf"
   integrity sha512-JCB8C6SnDoQf0cNycqd/35A7MjcnK+ZTqE7judS6o7utxUCg6imJg3QK2qzHKszlTjcj2cn+NwMB2i96ubpj7w==
 
-"@types/react-dom@^18.0.0", "@types/react-dom@^18.0.3":
+"@types/react-dom@^18.0.0":
   version "18.0.5"
   resolved "https://registry.yarnpkg.com/@types/react-dom/-/react-dom-18.0.5.tgz#330b2d472c22f796e5531446939eacef8378444a"
   integrity sha512-OWPWTUrY/NIrjsAPkAk1wW9LZeIjSvkXRhclsFO8CZcZGCOg2G0YZy4ft+rOyYxy8B7ui5iZzi9OkDebZ7/QSA==
   dependencies:
     "@types/react" "*"
 
-"@types/react@*", "@types/react@^18.0.8":
+"@types/react-dom@^19.0.0":
+  version "19.0.3"
+  resolved "https://registry.yarnpkg.com/@types/react-dom/-/react-dom-19.0.3.tgz#0804dfd279a165d5a0ad8b53a5b9e65f338050a4"
+  integrity sha512-0Knk+HJiMP/qOZgMyNFamlIjw9OFCsyC2ZbigmEEyXXixgre6IQpm/4V+r3qH4GC1JPvRJKInw+on2rV6YZLeA==
+
+"@types/react@*":
   version "18.0.12"
   resolved "https://registry.yarnpkg.com/@types/react/-/react-18.0.12.tgz#cdaa209d0a542b3fcf69cf31a03976ec4cdd8840"
   integrity sha512-duF1OTASSBQtcigUvhuiTB1Ya3OvSy+xORCiEf20H0P0lzx+/KeVsA99U5UjLXSbyo1DRJDlLKqTeM1ngosqtg==
@@ -2012,6 +2017,13 @@
     "@types/scheduler" "*"
     csstype "^3.0.2"
 
+"@types/react@^19.0.0":
+  version "19.0.8"
+  resolved "https://registry.yarnpkg.com/@types/react/-/react-19.0.8.tgz#7098e6159f2a61e4f4cef2c1223c044a9bec590e"
+  integrity sha512-9P/o1IGdfmQxrujGbIMDyYaaCykhLKc0NGCtYcECNUr9UAaDe4gwvV9bR6tvd5Br1SG0j+PBpbKr2UYY8CwqSw==
+  dependencies:
+    csstype "^3.0.2"
+
 "@types/resolve@1.17.1":
   version "1.17.1"
   resolved "https://registry.yarnpkg.com/@types/resolve/-/resolve-1.17.1.tgz#3afd6ad8967c77e4376c598a82ddd58f46ec45d6"
@@ -6276,7 +6288,7 @@ jest@^28.1.1:
     import-local "^3.0.2"
     jest-cli "^28.1.1"
 
-"js-tokens@^3.0.0 || ^4.0.0", js-tokens@^4.0.0:
+js-tokens@^4.0.0:
   version "4.0.0"
   resolved "https://registry.yarnpkg.com/js-tokens/-/js-tokens-4.0.0.tgz#19203fb59991df98e3a287050d4647cdeaf32499"
   integrity sha512-RdJUflcE3cUzKiMqQgsCu06FPu9UdIJO0beYbPhHN4k6apgJtifcoCtT9bcxOpYBtpD2kCM6Sbzg4CausW/PKQ==
@@ -6833,13 +6845,6 @@ log4js@^6.4.1:
     rfdc "^1.3.0"
     streamroller "^3.1.1"
 
-loose-envify@^1.1.0:
-  version "1.4.0"
-  resolved "https://registry.yarnpkg.com/loose-envify/-/loose-envify-1.4.0.tgz#71ee51fa7be4caec1a63839f7e682d8132d30caf"
-  integrity sha512-lyuxPGr/Wfhrlem2CL/UcnUc1zcqKAImBDzukY7Y5F/yQiNdko6+fRLevlw1HgMySw7f611UIY408EtxRSoK3Q==
-  dependencies:
-    js-tokens "^3.0.0 || ^4.0.0"
-
 lru-cache@^5.1.1:
   version "5.1.1"
   resolved "https://registry.yarnpkg.com/lru-cache/-/lru-cache-5.1.1.tgz#1da27e6710271947695daf6848e847f01d84b920"
@@ -8713,13 +8718,12 @@ react-app-polyfill@^3.0.0:
     regenerator-runtime "^0.13.9"
     whatwg-fetch "^3.6.2"
 
-react-dom@^18.0.0:
-  version "18.1.0"
-  resolved "https://registry.yarnpkg.com/react-dom/-/react-dom-18.1.0.tgz#7f6dd84b706408adde05e1df575b3a024d7e8a2f"
-  integrity sha512-fU1Txz7Budmvamp7bshe4Zi32d0ll7ect+ccxNu9FlObT605GOEB8BfO4tmRJ39R5Zj831VCpvQ05QPBW5yb+w==
+react-dom@^19.0.0:
+  version "19.0.0"
+  resolved "https://registry.yarnpkg.com/react-dom/-/react-dom-19.0.0.tgz#43446f1f01c65a4cd7f7588083e686a6726cfb57"
+  integrity sha512-4GV5sHFG0e/0AD4X+ySy6UJd3jVl1iNsNHdpad0qhABJ11twS3TTBnseqsKurKcsNqCEFeGL3uLpVChpIO3QfQ==
   dependencies:
-    loose-envify "^1.1.0"
-    scheduler "^0.22.0"
+    scheduler "^0.25.0"
 
 react-is@^17.0.1:
   version "17.0.2"
@@ -8731,12 +8735,10 @@ react-is@^18.0.0:
   resolved "https://registry.yarnpkg.com/react-is/-/react-is-18.1.0.tgz#61aaed3096d30eacf2a2127118b5b41387d32a67"
   integrity sha512-Fl7FuabXsJnV5Q1qIOQwx/sagGF18kogb4gpfcG4gjLBWO0WDiiz1ko/ExayuxE7InyQkBLkxRFG5oxY6Uu3Kg==
 
-react@^18.0.0:
-  version "18.1.0"
-  resolved "https://registry.yarnpkg.com/react/-/react-18.1.0.tgz#6f8620382decb17fdc5cc223a115e2adbf104890"
-  integrity sha512-4oL8ivCz5ZEPyclFQXaNksK3adutVS8l2xzZU0cqEFrE9Sb7fC0EFK5uEk74wIreL1DERyjvsU915j1pcT2uEQ==
-  dependencies:
-    loose-envify "^1.1.0"
+react@^19.0.0:
+  version "19.0.0"
+  resolved "https://registry.yarnpkg.com/react/-/react-19.0.0.tgz#6e1969251b9f108870aa4bff37a0ce9ddfaaabdd"
+  integrity sha512-V8AVnmPIICiWpGfm6GLzCR/W5FXLchHop40W4nXBmdlEceh16rCN8O8LNWm5bh5XUX91fh7KpA+W0TgMKmgTpQ==
 
 read-cmd-shim@^3.0.0:
   version "3.0.0"
@@ -9116,12 +9118,10 @@ saxes@^5.0.1:
   dependencies:
     xmlchars "^2.2.0"
 
-scheduler@^0.22.0:
-  version "0.22.0"
-  resolved "https://registry.yarnpkg.com/scheduler/-/scheduler-0.22.0.tgz#83a5d63594edf074add9a7198b1bae76c3db01b8"
-  integrity sha512-6QAm1BgQI88NPYymgGQLCZgvep4FyePDWFpXVK+zNSUgHwlqpJy8VEh8Et0KxTACS4VWwMousBElAZOH9nkkoQ==
-  dependencies:
-    loose-envify "^1.1.0"
+scheduler@^0.25.0:
+  version "0.25.0"
+  resolved "https://registry.yarnpkg.com/scheduler/-/scheduler-0.25.0.tgz#336cd9768e8cceebf52d3c80e3dcf5de23e7e015"
+  integrity sha512-xFVuu11jh+xcO7JOAGJNOXld8/TcEHK/4CituBUeUb5hqxJLj9YuemAEuvm9gQ/+pgXYfbQuqAkiYu+u7YEsNA==
 
 schema-utils@^1.0.0:
   version "1.0.0"