diff --git a/cli/src/command/poll.rs b/cli/src/command/poll.rs
index 3c201fb..dd35ac9 100644
--- a/cli/src/command/poll.rs
+++ b/cli/src/command/poll.rs
@@ -21,7 +21,7 @@ use std::sync::Mutex;
 use serde::Deserialize;
 use tabled::Tabled;
 
-use crate::formatter::{error_without_trace, info, success, success_table};
+use crate::formatter::{error_without_trace, info, success, success_table, warn_without_trace};
 
 const DEFAULT_CONFIG: &str = "./collector.toml";
 
@@ -52,9 +52,9 @@ pub struct Source {
     pub env: BTreeMap<String, String>,
     #[serde(default)]
     pub layout: Layout,
-    /// Modules this source is expected to produce. Polling fails if any of
-    /// them is missing afterwards, so a producer that silently stops emitting
-    /// a module cannot slip into a release.
+    /// Modules this source is expected to produce. Polling warns, rather
+    /// than failing, if one of them is missing afterwards -- useful while a
+    /// module is still under development and not always exported yet.
     pub modules: Vec<String>,
 }
 
@@ -169,10 +169,11 @@ pub fn poll(
         for module in &source.modules {
             let module_dir = out_dir.join(module);
             if !module_dir.is_dir() {
-                fail(format!(
-                    "`{}` finished without producing the module `{}` it declares in {}.",
+                warn_without_trace(format!(
+                    "`{}` finished without producing the module `{}` it declares in {}. Skipping it.",
                     source.name, module, config_path
                 ));
+                continue;
             }
 
             verify_module(&module_dir, &source.name, module);
diff --git a/cli/src/formatter.rs b/cli/src/formatter.rs
index 03aab8c..0c00021 100644
--- a/cli/src/formatter.rs
+++ b/cli/src/formatter.rs
@@ -30,6 +30,10 @@ pub fn error_without_trace(string: String) {
     println!("\n{}: {}", "error".red(), string)
 }
 
+pub fn warn_without_trace(string: String) {
+    println!("\n{}: {}", "warning".yellow(), string)
+}
+
 pub fn error_highlight(highlight: String, string: String) {
     println!("{} {}", highlight.red(), string);
 }
diff --git a/collector.toml b/collector.toml
index b86f56c..acfc6af 100644
--- a/collector.toml
+++ b/collector.toml
@@ -18,6 +18,10 @@ out = "./definitions"
 #   "tree"   -- writes one directory per module into the directory it is
 #               given, so it is handed `<out>`. This is what a taurus export
 #               built on `build_modules()` does.
+#
+# `modules` lists the modules a source is expected to produce. Polling warns
+# and skips one that is missing rather than failing, so a module still under
+# development doesn't need to be exported every time.
 
 [[source]]
 name = "taurus"