diff --git a/src/bindgen/cargo/cargo.rs b/src/bindgen/cargo/cargo.rs
index 964b37b..1718a3d 100644
--- a/src/bindgen/cargo/cargo.rs
+++ b/src/bindgen/cargo/cargo.rs
@@ -13,10 +13,10 @@ use crate::bindgen::error::Error;
 use crate::bindgen::ir::Cfg;
 
 /// Parse a dependency string used in Cargo.lock
-fn parse_dep_string(dep_string: &str) -> (&str, &str) {
+fn parse_dep_string(dep_string: &str) -> (&str, Option<&str>) {
     let split: Vec<&str> = dep_string.split_whitespace().collect();
 
-    (split[0], split[1])
+    (split[0], split.get(1).cloned())
 }
 
 /// A collection of metadata for a library from cargo.
@@ -99,14 +99,25 @@ impl Cargo {
 
         // Find the dependencies listing in the lockfile
         if let Some(ref root) = lock.root {
-            if root.name == package.name && root.version == package.version {
+            // If the version is not on the lockfile then it shouldn't be
+            // ambiguous.
+            if root.name == package.name
+                && package
+                    .version
+                    .as_ref()
+                    .map_or(true, |v| *v == root.version)
+            {
                 dependencies = root.dependencies.as_ref();
             }
         }
         if dependencies.is_none() {
             if let Some(ref lock_packages) = lock.package {
                 for lock_package in lock_packages {
-                    if lock_package.name == package.name && lock_package.version == package.version
+                    if lock_package.name == package.name
+                        && package
+                            .version
+                            .as_ref()
+                            .map_or(true, |v| *v == lock_package.version)
                     {
                         dependencies = lock_package.dependencies.as_ref();
                         break;
@@ -134,7 +145,7 @@ impl Cargo {
 
                 let package_ref = PackageRef {
                     name: dep_name.to_owned(),
-                    version: dep_version.to_owned(),
+                    version: dep_version.map(|v| v.to_owned()),
                 };
 
                 (package_ref, cfg)
@@ -202,7 +213,7 @@ impl Cargo {
         cargo_expand::expand(
             &self.manifest_path,
             &package.name,
-            &package.version,
+            package.version.as_ref().map(|v| &**v),
             self.clean,
             expand_all_features,
             expand_default_features,
diff --git a/src/bindgen/cargo/cargo_expand.rs b/src/bindgen/cargo/cargo_expand.rs
index 86e4515..4697223 100644
--- a/src/bindgen/cargo/cargo_expand.rs
+++ b/src/bindgen/cargo/cargo_expand.rs
@@ -60,7 +60,7 @@ impl error::Error for Error {
 pub fn expand(
     manifest_path: &Path,
     crate_name: &str,
-    version: &str,
+    version: Option<&str>,
     use_tempdir: bool,
     expand_all_features: bool,
     expand_default_features: bool,
@@ -109,7 +109,12 @@ pub fn expand(
         cmd.arg("--no-default-features");
     }
     cmd.arg("-p");
-    cmd.arg(&format!("{}:{}", crate_name, version));
+    let mut package = crate_name.to_owned();
+    if let Some(version) = version {
+        package.push_str(":");
+        package.push_str(version);
+    }
+    cmd.arg(&package);
     cmd.arg("--verbose");
     cmd.arg("--");
     cmd.arg("-Z");
diff --git a/src/bindgen/cargo/cargo_lock.rs b/src/bindgen/cargo/cargo_lock.rs
index bc59071..9d68a3b 100644
--- a/src/bindgen/cargo/cargo_lock.rs
+++ b/src/bindgen/cargo/cargo_lock.rs
@@ -39,7 +39,7 @@ pub struct Lock {
 pub struct Package {
     pub name: String,
     pub version: String,
-    /// A list of dependencies formatted like "NAME VERSION REGISTRY-OPT"
+    /// A list of dependencies formatted like "NAME VERSION-OPT REGISTRY-OPT"
     pub dependencies: Option<Vec<String>>,
 }
 
diff --git a/src/bindgen/cargo/cargo_metadata.rs b/src/bindgen/cargo/cargo_metadata.rs
index 681acc8..2745029 100644
--- a/src/bindgen/cargo/cargo_metadata.rs
+++ b/src/bindgen/cargo/cargo_metadata.rs
@@ -36,7 +36,7 @@ pub struct Metadata {
 #[derive(Clone, Debug, Hash, Eq, PartialEq, Serialize, Deserialize)]
 pub struct PackageRef {
     pub name: String,
-    pub version: String,
+    pub version: Option<String>,
 }
 
 #[derive(Clone, Deserialize, Debug)]
diff --git a/src/bindgen/parser.rs b/src/bindgen/parser.rs
index 017fbdd..f75ca65 100644
--- a/src/bindgen/parser.rs
+++ b/src/bindgen/parser.rs
@@ -52,7 +52,7 @@ pub fn parse_src(src_file: &FilePath, config: &Config) -> ParseResult {
 
     let pkg_ref = PackageRef {
         name: mod_name.to_owned(),
-        version: "0.0.0".to_owned(),
+        version: None,
     };
 
     context.parse_mod(&pkg_ref, src_file)?;