diff --git a/aws-lc-rs/README.md b/aws-lc-rs/README.md
index d4ebc8b969a..e366c937fc6 100644
--- a/aws-lc-rs/README.md
+++ b/aws-lc-rs/README.md
@@ -90,7 +90,7 @@ detected in the build environment *it is used* to compile the assembly files. Ho
 if a NASM assembler is not available, and the "fips" feature is not enabled, then the build fails unless one of the following conditions are true:
 
 * You are building for `x86-64` and either:
-   * The `AWS_LC_SYS_PREBUILT_NASM` environment variable is found and has a value of "1" (or the value is empty); OR
+   * The `AWS_LC_SYS_PREBUILT_NASM` environment variable is found and has a value of "1"; OR
    * `AWS_LC_SYS_PREBUILT_NASM` is *not found* in the environment AND the "prebuilt-nasm" feature has been enabled.
 
 If the above cases apply, then the crate provided prebuilt NASM objects will be used for the build. To prevent usage of prebuilt NASM
diff --git a/aws-lc-rs/src/lib.rs b/aws-lc-rs/src/lib.rs
index 1e75b8eb149..5632ec4c9d1 100644
--- a/aws-lc-rs/src/lib.rs
+++ b/aws-lc-rs/src/lib.rs
@@ -91,7 +91,7 @@
 //! if a NASM assembler is not available, and the "fips" feature is not enabled, then the build fails unless one of the following conditions are true:
 //!
 //! * You are building for `x86-64` and either:
-//!    * The `AWS_LC_SYS_PREBUILT_NASM` environment variable is found and has a value of "1" (or the value is empty); OR
+//!    * The `AWS_LC_SYS_PREBUILT_NASM` environment variable is found and has a value of "1"; OR
 //!    * `AWS_LC_SYS_PREBUILT_NASM` is *not found* in the environment AND the "prebuilt-nasm" feature has been enabled.
 //!
 //! If the above cases apply, then the crate provided prebuilt NASM objects will be used for the build. To prevent usage of prebuilt NASM
diff --git a/aws-lc-sys/README.md b/aws-lc-sys/README.md
index 723d1e5a495..1f96e3585ca 100644
--- a/aws-lc-sys/README.md
+++ b/aws-lc-sys/README.md
@@ -43,7 +43,7 @@ if a NASM assembler is not available, and the "fips" feature is not enabled, the
 following conditions are true:
 
 * You are building for `x86-64` and either:
-    * The `AWS_LC_SYS_PREBUILT_NASM` environment variable is found and has a value of "1" (or the value is empty); OR
+    * The `AWS_LC_SYS_PREBUILT_NASM` environment variable is found and has a value of "1"; OR
     * `AWS_LC_SYS_PREBUILT_NASM` is *not found* in the environment AND the "prebuilt-nasm" feature has been enabled.
 
 If the above cases apply, then the crate provided prebuilt NASM objects will be used for the build. To prevent usage of
diff --git a/aws-lc-sys/builder/main.rs b/aws-lc-sys/builder/main.rs
index 0773085b4e2..3fae1e4448b 100644
--- a/aws-lc-sys/builder/main.rs
+++ b/aws-lc-sys/builder/main.rs
@@ -77,21 +77,28 @@ fn option_env<N: AsRef<str>>(name: N) -> Option<String> {
 fn env_var_to_bool(name: &str) -> Option<bool> {
     let build_type_result = option_env(name);
     if let Some(env_var_value) = build_type_result {
-        eprintln!("{name}={env_var_value}");
-        // If the environment variable is set, we ignore every other factor.
+        eprintln!("Evaluating: {name}='{env_var_value}'");
+
         let env_var_value = env_var_value.to_lowercase();
         if env_var_value.starts_with('0')
             || env_var_value.starts_with('n')
             || env_var_value.starts_with("off")
+            || env_var_value.starts_with('f')
         {
-            Some(false)
-        } else {
-            // Otherwise, if the variable is set, assume true
-            Some(true)
+            eprintln!("Parsed: {name}=false");
+            return Some(false);
         }
-    } else {
-        None
+        if env_var_value.starts_with(|c: char| c.is_ascii_digit())
+            || env_var_value.starts_with('y')
+            || env_var_value.starts_with("on")
+            || env_var_value.starts_with('t')
+        {
+            eprintln!("Parsed: {name}=true");
+            return Some(true);
+        }
+        eprintln!("Parsed: {name}=unknown");
     }
+    None
 }
 
 impl Default for OutputLibType {
diff --git a/book/src/requirements/windows.md b/book/src/requirements/windows.md
index ffe3a46742e..fe52787465d 100644
--- a/book/src/requirements/windows.md
+++ b/book/src/requirements/windows.md
@@ -49,7 +49,7 @@ if a NASM assembler is not available, and the "fips" feature is not enabled, the
 following conditions are true:
 
 * You are building for `x86-64` and either:
-    * The `AWS_LC_SYS_PREBUILT_NASM` environment variable is found and has a value of "1" (or the value is empty); OR
+    * The `AWS_LC_SYS_PREBUILT_NASM` environment variable is found and has a value of "1"; OR
     * `AWS_LC_SYS_PREBUILT_NASM` is *not found* in the environment AND the "prebuilt-nasm" feature has been enabled.
 
 If the above cases apply, then the crate provided prebuilt NASM objects will be used for the build. To prevent usage of