diff --git a/src/input_widget.rs b/src/input_widget.rs
index fdee459..ea197d5 100644
--- a/src/input_widget.rs
+++ b/src/input_widget.rs
@@ -142,10 +142,8 @@ extern "C" fn resize_callback(data: *mut sys::ImGuiInputTextCallbackData) -> c_i
                 if requested_size > buffer.capacity_with_nul() {
                     // Refresh the buffer's length to take into account changes made by dear imgui.
                     buffer.refresh_len();
-                    // Add 1 to include the null terminator, so that reserve sees the right length.
-                    // After we're done we'll call refresh_len, so this won't be visible to the user.
-                    buffer.0.set_len(buffer.0.len() + 1);
                     buffer.reserve(requested_size - buffer.0.len());
+                    debug_assert!(buffer.capacity_with_nul() >= requested_size);
                     (*data).Buf = buffer.as_mut_ptr();
                     (*data).BufDirty = true;
                 }
diff --git a/src/string.rs b/src/string.rs
index c9be95c..135140a 100644
--- a/src/string.rs
+++ b/src/string.rs
@@ -26,7 +26,11 @@ pub struct ImString(pub(crate) Vec<u8>);
 impl ImString {
     /// Creates a new `ImString` from an existing string.
     pub fn new<T: Into<String>>(value: T) -> ImString {
-        unsafe { ImString::from_utf8_unchecked(value.into().into_bytes()) }
+        unsafe {
+            let mut s = ImString::from_utf8_unchecked(value.into().into_bytes());
+            s.refresh_len();
+            s
+        }
     }
     /// Creates a new empty `ImString` with a particular capacity
     pub fn with_capacity(capacity: usize) -> ImString {
@@ -57,9 +61,12 @@ impl ImString {
     }
     /// Appends a given string slice to the end of this `ImString`
     pub fn push_str(&mut self, string: &str) {
-        self.refresh_len();
-        self.0.extend_from_slice(string.as_bytes());
+        self.0.pop();
+        self.0.extend(string.bytes());
         self.0.push(b'\0');
+        unsafe {
+            self.refresh_len();
+        }
     }
     /// Returns the capacity of this `ImString` in bytes
     pub fn capacity(&self) -> usize {
@@ -81,22 +88,25 @@ impl ImString {
     pub fn reserve_exact(&mut self, additional: usize) {
         self.0.reserve_exact(additional);
     }
+    /// Returns a raw pointer to the underlying buffer
     pub fn as_ptr(&self) -> *const c_char {
-        self.0.as_ptr() as *const _
+        self.0.as_ptr() as *const c_char
     }
-    pub fn as_mut_ptr(&mut self) -> *mut c_char {
-        self.0.as_mut_ptr() as *mut _
-    }
-    /// Updates the buffer length based on the current contents.
+    /// Returns a raw mutable pointer to the underlying buffer.
     ///
-    /// Dear imgui accesses pointers directly, so the length doesn't get updated when the contents
-    /// change. This is normally OK, because Deref to ImStr always calculates the slice length
-    /// based on contents. However, we need to refresh the length in some ImString functions.
-    pub(crate) fn refresh_len(&mut self) {
-        let len = self.to_str().len();
-        unsafe {
-            self.0.set_len(len);
-        }
+    /// If the underlying data is modified, `refresh_len` *must* be called afterwards.
+    pub fn as_mut_ptr(&mut self) -> *mut c_char {
+        self.0.as_mut_ptr() as *mut c_char
+    }
+    /// Updates the underlying buffer length based on the current contents.
+    ///
+    /// This function *must* be called if the underlying data is modified via a pointer
+    /// obtained by `as_mut_ptr`.
+    pub unsafe fn refresh_len(&mut self) {
+        let len = CStr::from_ptr(self.0.as_ptr() as *const c_char)
+            .to_bytes_with_nul()
+            .len();
+        self.0.set_len(len);
     }
 }
 
@@ -108,7 +118,7 @@ impl<'a> Default for ImString {
 
 impl From<String> for ImString {
     fn from(s: String) -> ImString {
-        unsafe { ImString::from_utf8_unchecked(s.into_bytes()) }
+        ImString::new(s)
     }
 }
 
@@ -177,7 +187,8 @@ impl Deref for ImString {
     type Target = ImStr;
     fn deref(&self) -> &ImStr {
         // as_ptr() is used, because we need to look at the bytes to figure out the length
-        // self.0.len() is incorrect, because there might be more than one nul byte in the end
+        // self.0.len() is incorrect, because there might be more than one nul byte in the end, or
+        // some interior nuls in the data
         unsafe {
             &*(CStr::from_ptr(self.0.as_ptr() as *const c_char) as *const CStr as *const ImStr)
         }
@@ -309,14 +320,14 @@ fn test_imstring_refresh_len() {
         *ptr = b'\0';
     }
     assert_eq!(s.0, b"tez\0ing\0");
-    unsafe {  s.refresh_len() };
+    unsafe { s.refresh_len() };
     assert_eq!(s.0, b"tez\0");
 }
 
 #[test]
 fn test_imstring_interior_nul() {
     let s = ImString::new("test\0ohno");
-    assert_eq!(s.0, b"test\0ohno\0");
+    assert_eq!(s.0, b"test\0");
     assert_eq!(s.to_str(), "test");
     assert!(!s.is_empty());