diff --git a/azure-pipelines.yml b/azure-pipelines.yml
index 6e5788d4a..936d1dd90 100644
--- a/azure-pipelines.yml
+++ b/azure-pipelines.yml
@@ -6,7 +6,7 @@
 jobs:
 - job: Linux
   pool:
-    vmImage: 'ubuntu-16.04'
+    vmImage: 'ubuntu-18.04'
   strategy:
     matrix:
       node_12_x:
@@ -83,7 +83,7 @@ jobs:
   - Windows
   condition: and(succeeded(), eq(variables['Build.SourceBranch'], 'refs/heads/main'))
   pool:
-    vmImage: 'ubuntu-16.04'
+    vmImage: 'ubuntu-18.04'
   steps:
   - task: NodeTool@0
     inputs:
diff --git a/src/win/conpty.cc b/src/win/conpty.cc
index 4500f6ef0..c5752572b 100644
--- a/src/win/conpty.cc
+++ b/src/win/conpty.cc
@@ -289,6 +289,13 @@ static NAN_METHOD(PtyConnect) {
   const v8::Local<v8::Array> envValues = info[3].As<v8::Array>();
   const v8::Local<v8::Function> exitCallback = v8::Local<v8::Function>::Cast(info[4]);
 
+  // Fetch pty handle from ID and start process
+  pty_baton* handle = get_pty_baton(id);
+  if (!handle) {
+    Nan::ThrowError("Invalid pty handle");
+    return;
+  }
+
   // Prepare command line
   std::unique_ptr<wchar_t[]> mutableCommandline = std::make_unique<wchar_t[]>(cmdline.length() + 1);
   HRESULT hr = StringCchCopyW(mutableCommandline.get(), cmdline.length() + 1, cmdline.c_str());
@@ -311,9 +318,6 @@ static NAN_METHOD(PtyConnect) {
   auto envV = vectorFromString(env);
   LPWSTR envArg = envV.empty() ? nullptr : envV.data();
 
-  // Fetch pty handle from ID and start process
-  pty_baton* handle = get_pty_baton(id);
-
   BOOL success = ConnectNamedPipe(handle->hIn, nullptr);
   success = ConnectNamedPipe(handle->hOut, nullptr);
 
@@ -396,15 +400,17 @@ static NAN_METHOD(PtyResize) {
 
   const pty_baton* handle = get_pty_baton(id);
 
-  HANDLE hLibrary = LoadLibraryExW(L"kernel32.dll", 0, 0);
-  bool fLoadedDll = hLibrary != nullptr;
-  if (fLoadedDll)
-  {
-    PFNRESIZEPSEUDOCONSOLE const pfnResizePseudoConsole = (PFNRESIZEPSEUDOCONSOLE)GetProcAddress((HMODULE)hLibrary, "ResizePseudoConsole");
-    if (pfnResizePseudoConsole)
+  if (handle != nullptr) {
+    HANDLE hLibrary = LoadLibraryExW(L"kernel32.dll", 0, 0);
+    bool fLoadedDll = hLibrary != nullptr;
+    if (fLoadedDll)
     {
-      COORD size = {cols, rows};
-      pfnResizePseudoConsole(handle->hpc, size);
+      PFNRESIZEPSEUDOCONSOLE const pfnResizePseudoConsole = (PFNRESIZEPSEUDOCONSOLE)GetProcAddress((HMODULE)hLibrary, "ResizePseudoConsole");
+      if (pfnResizePseudoConsole)
+      {
+        COORD size = {cols, rows};
+        pfnResizePseudoConsole(handle->hpc, size);
+      }
     }
   }
 
@@ -424,18 +430,20 @@ static NAN_METHOD(PtyKill) {
 
   const pty_baton* handle = get_pty_baton(id);
 
-  HANDLE hLibrary = LoadLibraryExW(L"kernel32.dll", 0, 0);
-  bool fLoadedDll = hLibrary != nullptr;
-  if (fLoadedDll)
-  {
-    PFNCLOSEPSEUDOCONSOLE const pfnClosePseudoConsole = (PFNCLOSEPSEUDOCONSOLE)GetProcAddress((HMODULE)hLibrary, "ClosePseudoConsole");
-    if (pfnClosePseudoConsole)
+  if (handle != nullptr) {
+    HANDLE hLibrary = LoadLibraryExW(L"kernel32.dll", 0, 0);
+    bool fLoadedDll = hLibrary != nullptr;
+    if (fLoadedDll)
     {
-      pfnClosePseudoConsole(handle->hpc);
+      PFNCLOSEPSEUDOCONSOLE const pfnClosePseudoConsole = (PFNCLOSEPSEUDOCONSOLE)GetProcAddress((HMODULE)hLibrary, "ClosePseudoConsole");
+      if (pfnClosePseudoConsole)
+      {
+        pfnClosePseudoConsole(handle->hpc);
+      }
     }
-  }
 
-  CloseHandle(handle->hShell);
+    CloseHandle(handle->hShell);
+  }
 
   return info.GetReturnValue().SetUndefined();
 }
diff --git a/src/win/path_util.cc b/src/win/path_util.cc
index 4e69f3091..841b47cae 100644
--- a/src/win/path_util.cc
+++ b/src/win/path_util.cc
@@ -52,7 +52,7 @@ std::wstring get_shell_path(std::wstring filename) {
 
   const wchar_t *filename_ = filename.c_str();
 
-  for (int i = 0; i < paths.size(); ++i) {
+  for (size_t i = 0; i < paths.size(); ++i) {
     std::wstring path = paths[i];
     wchar_t searchPath[MAX_PATH];
     ::PathCombineW(searchPath, const_cast<wchar_t*>(path.c_str()), filename_);